Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurement - Summary of Assets Measured at Fair Value on Recurring Basis (Detail)

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Fair Value Measurement - Summary of Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis $ 402,889 $ 390,679
Other [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 16,271 16,139
Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 386,618 374,540
Cash and Cash Equivalents [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 11 121
Cash and Cash Equivalents [Member] | Money Market Funds [Member] | Other [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 11 121
Cash and Cash Equivalents [Member] | U.S. Treasuries [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis   100
Cash and Cash Equivalents [Member] | U.S. Treasuries [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis   100
Investments [Member] | U.S. Treasuries [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 841 399
Investments [Member] | U.S. Treasuries [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 841 399
Investments [Member] | Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 582 817
Investments [Member] | Corporate Bonds [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 582 817
Investments [Member] | Mortgage Backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 532 572
Investments [Member] | Mortgage Backed Securities [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 532 572
Investments [Member] | U.S. Government Agency Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 109 65
Investments [Member] | U.S. Government Agency Securities [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 109 65
Investments [Member] | Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 5 10
Investments [Member] | Municipal Bonds [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 5 10
Investments [Member] | Asset Backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 10 10
Investments [Member] | Asset Backed Securities [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 10 10
Restricted Cash and Investments [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 16,260 16,018
Restricted Cash and Investments [Member] | Money Market Funds [Member] | Other [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 16,260 16,018
Restricted Cash and Investments [Member] | U.S. Treasuries [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 55,102 45,690
Restricted Cash and Investments [Member] | U.S. Treasuries [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 55,102 45,690
Restricted Cash and Investments [Member] | Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 182,192 183,855
Restricted Cash and Investments [Member] | Corporate Bonds [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 182,192 183,855
Restricted Cash and Investments [Member] | Mortgage Backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 95,044 85,645
Restricted Cash and Investments [Member] | Mortgage Backed Securities [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 95,044 85,645
Restricted Cash and Investments [Member] | U.S. Government Agency Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 42,146 37,946
Restricted Cash and Investments [Member] | U.S. Government Agency Securities [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 42,146 37,946
Restricted Cash and Investments [Member] | Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 250 458
Restricted Cash and Investments [Member] | Municipal Bonds [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 250 458
Restricted Cash and Investments [Member] | Asset Backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 74  
Restricted Cash and Investments [Member] | Asset Backed Securities [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 74  
Restricted Cash and Investments [Member] | Commercial Paper [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 4,995 18,973
Restricted Cash and Investments [Member] | Commercial Paper [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 4,995 $ 18,973
Restricted Cash and Investments [Member] | Supranational Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 4,736  
Restricted Cash and Investments [Member] | Supranational Bonds [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis $ 4,736