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Investments - Summary of Securities Available for Sale (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Investments Debt And Equity Securities [Abstract]    
Proceeds from sales and maturities (fair value) $ 22,329 $ 52,366
Gross realized gains   685
Gross realized losses $ (63) $ (18)