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Investments - Gross Realized Gains and Losses on Sale of Securities (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Investments Debt And Equity Securities [Abstract]        
Gross realized gains $ 1,512 $ 1,517 $ 2,230 $ 3,881
Gross realized losses $ (122) $ (407) $ (370) $ (1,043)