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Investment Securities - Proceeds from Sale, Gross Gains (Losses) Realized on Sales, Maturities and Other-Than-Temporary Impairment Charges Related to Securities Available for Sale (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Gain (Loss) on Investments [Line Items]      
Proceeds from sale $ 143,660 $ 55,744 $ 88,054
Maturities, calls and impairment:      
Net gains (losses) and impairment 5,040 617 (153)
Calls And Maturities [Member]      
Sales Transactions:      
Gross losses (60) 0 0
Maturities, calls and impairment:      
Gross gains 5,057 589 131
Other-than-temporary impairment 0 0 0
Total gain (losses) maturities and impairment 4,997 589 131
Sales [Member]      
Sales Transactions:      
Available-for-sale Securities, Gross Realized Losses 359 305 0
Gross losses (316) (277) (284)
Total gain (losses) from sales transactions $ 43 $ 28 $ (284)