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Investment Securities - Summary of Interest Income Earned on Investment Securities (Detail) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Net Investment Income [Line Items] |  |  |  |
Total interest income from available-for-sale securities | $ 11,697 | $ 13,007 | $ 23,565 | $ 25,647
Total interest income from held-to-maturity securities | 4,807 | 5,323 | 9,572 | 10,830
Total investment income | 16,504 | 18,330 | 33,137 | 36,477
Investment securities available-for-sale [Member] |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Taxable | 11,290 | 12,420 | 22,735 | 24,346
Tax-advantaged | 407 | 587 | 830 | 1,301
Investment securities held-to-maturity [Member] |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Taxable | 3,048 | 3,203 | 5,926 | 6,480
Tax-advantaged | $ 1,759 | $ 2,120 | $ 3,646 | $ 4,350

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