Quarterly report pursuant to Section 13 or 15(d)

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

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Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost $ 410,345 $ 392,615
Gross Unrealized Gains (Losses) (7,456) (1,936)
Recorded Basis 402,889 390,679
Investments Current [Member] | Cash and Cash Equivalents [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 11 221
Recorded Basis 11 221
Investments Current [Member] | Cash and Cash Equivalents [Member] | Money Market Funds [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 11 121
Recorded Basis 11 121
Investments Current [Member] | Cash and Cash Equivalents [Member] | U.S. Treasuries [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost   100
Recorded Basis   100
Investments Current [Member] | Investments [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 473 674
Gross Unrealized Gains (Losses) (1)  
Recorded Basis 472 674
Investments Current [Member] | Investments [Member] | U.S. Treasuries [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 200 199
Recorded Basis 200 199
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 208 400
Gross Unrealized Gains (Losses) (1)  
Recorded Basis 207 400
Investments Current [Member] | Investments [Member] | U.S. Government Agency Securities [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 60 65
Recorded Basis 60 65
Investments Current [Member] | Investments [Member] | Municipal Bonds [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 5 10
Recorded Basis 5 10
Investments Noncurrent [Member] | Investments [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 1,631 1,208
Gross Unrealized Gains (Losses) (24) (9)
Recorded Basis 1,607 1,199
Investments Noncurrent [Member] | Investments [Member] | U.S. Treasuries [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 645 202
Gross Unrealized Gains (Losses) (4) (2)
Recorded Basis 641 200
Investments Noncurrent [Member] | Investments [Member] | Corporate Bonds [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 383 419
Gross Unrealized Gains (Losses) (8) (2)
Recorded Basis 375 417
Investments Noncurrent [Member] | Investments [Member] | U.S. Government Agency Securities [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 50  
Gross Unrealized Gains (Losses) (1)  
Recorded Basis 49  
Investments Noncurrent [Member] | Investments [Member] | Mortgage Backed Securities [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 543 577
Gross Unrealized Gains (Losses) (11) (5)
Recorded Basis 532 572
Investments Noncurrent [Member] | Investments [Member] | Asset Backed Securities [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 10 10
Recorded Basis 10 10
Investments Noncurrent [Member] | Restricted Cash and Investments [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 408,230 390,512
Gross Unrealized Gains (Losses) (7,431) (1,927)
Recorded Basis 400,799 388,585
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Money Market Funds [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 16,260 16,018
Recorded Basis 16,260 16,018
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | U.S. Treasuries [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 55,379 45,833
Gross Unrealized Gains (Losses) (277) (143)
Recorded Basis 55,102 45,690
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Corporate Bonds [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 186,085 184,808
Gross Unrealized Gains (Losses) (3,893) (953)
Recorded Basis 182,192 183,855
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | U.S. Government Agency Securities [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 43,164 38,168
Gross Unrealized Gains (Losses) (1,018) (222)
Recorded Basis 42,146 37,946
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Municipal Bonds [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 251 472
Gross Unrealized Gains (Losses) (1) (14)
Recorded Basis 250 458
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Mortgage Backed Securities [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 97,259 86,240
Gross Unrealized Gains (Losses) (2,215) (595)
Recorded Basis 95,044 85,645
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Asset Backed Securities [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 75  
Gross Unrealized Gains (Losses) (1)  
Recorded Basis 74  
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Commercial Paper [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 4,995 18,973
Recorded Basis 4,995 $ 18,973
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Supranational Bonds [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 4,762  
Gross Unrealized Gains (Losses) (26)  
Recorded Basis $ 4,736