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Investments - Summary of Securities Available for Sale (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Investments Debt And Equity Securities [Abstract]    
Sales proceeds (fair value) $ 146,576 $ 92,335
Gross realized gains 2,329 1,553
Gross realized losses $ (35) $ (493)