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Summary of Gross Proceeds and Gross Realized Gains from Sale of Available-for-Sale Securities (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Gain (Loss) on Investments [Line Items]    
Gross proceeds $ 139,002 $ 104,663
Gross realized gains $ 1,979 $ 163