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Summary of Gross Proceeds and Gross Realized Gains from Sale of Available-for-Sale Securities (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Gain (Loss) on Investments [Line Items]      
Gross proceeds $ 322,038 $ 278,689 $ 333,725
Gross realized gains $ 3,128 1,159 $ 6,186
Gross realized losses   $ 4