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Summary of Gross Proceeds, Gross Realized Gains, and Gross Realized Losses from Sale of Available-for-Sale Securities (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Gain (Loss) on Investments [Line Items]      
Gross proceeds $ 278,689 $ 333,725 $ 631,802
Gross realized gains 1,159 $ 6,186 9,529
Gross realized losses $ 4   $ 45