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Condensed Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]        
Unrealized gains (losses) on investments, tax $ 376 $ (396) $ 2,935 $ (2,325)