Quarterly report pursuant to Section 13 or 15(d)

Condensed Consolidated Statements of Comprehensive Income (Parenthetical)

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Condensed Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]    
Unrealized losses on investments, tax $ (1,555) $ (14)