Investment Securities - Summary of Interest Income Earned on Investment Securities (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Investment Income [Line Items]
Total interest income from available-for-sale securities	$ 49,778	$ 47,702	$ 63,190
Total interest income from held-to-maturity securities	21,015	20,227	9,018
Total investment income	70,793	67,929	72,208
Investment securities available-for-sale [Member]
Net Investment Income [Line Items]
Taxable	47,596	43,538	48,854
Tax-advantaged	2,182	4,164	14,336
Investment securities held-to-maturity [Member]
Net Investment Income [Line Items]
Taxable	12,558	10,183	4,451
Tax-advantaged	$ 8,457	$ 10,044	$ 4,567

Amount of operating dividend and interest income, including amortization and accretion of premiums and discounts, on securities.

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Amount of operating dividend and interest income, including amortization and accretion of premiums and discounts, on available-for-sale securities.

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Amount of operating interest income, including amortization and accretion of premiums and discounts, on held-to-maturity securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.2)
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82895926&loc=d3e24721-111560

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

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Amount of operating interest income, including amortization and accretion of premiums and discounts, on securities subject to tax benefits. Excludes operating interest income on securities exempt from state, federal and other income tax.

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Amount of operating interest income, including amortization and accretion of premiums and discounts, on securities subject to state, federal and other income tax.

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