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Investments - Summary of Securities Available for Sale (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Investments Debt And Equity Securities [Abstract]        
Proceeds from sales and maturities (fair value) $ 67,818 $ 23,744 $ 175,440 $ 188,896
Gross realized gains 849 107 2,662 1,369
Gross realized losses $ (46) $ (6) $ (212) $ (25)