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Investment Securities - Summary of Interest Income Earned on Investment Securities (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Net Investment Income [Line Items]        
Total interest income from available-for-sale securities $ 11,521 $ 12,240 $ 35,086 $ 37,887
Total interest income from held-to-maturity securities 4,666 5,184 14,238 16,014
Total investment income 16,187 17,424 49,324 53,901
Investment securities available-for-sale [Member]        
Net Investment Income [Line Items]        
Taxable 11,126 11,767 33,861 36,113
Tax-advantaged 395 473 1,225 1,774
Investment securities held-to-maturity [Member]        
Net Investment Income [Line Items]        
Taxable 2,961 3,111 8,887 9,591
Tax-advantaged $ 1,705 $ 2,073 $ 5,351 $ 6,423