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Summary of Gross Proceeds and Gross Realized Gains and Losses from Sale of Available-for-Sale Securities (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Gain (Loss) on Investments [Line Items]      
Gross proceeds $ 453,878 $ 322,038 $ 278,689
Gross realized gains 3,848 $ 3,128 1,159
Gross realized losses $ 860   $ 4