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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

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