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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Gain (Loss) on Investments [Line Items]      
Gross proceeds $ 361,311 $ 278,539 $ 453,878
Gross realized gains $ 5,445 967 3,848
Gross realized losses   $ 981 $ 860