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 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]        
Unrealized gains (losses) on investments, tax $ (396) $ 51 $ (2,325) $ 128