Investment Securities - Summary of Interest Income Earned on Investment Securities (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Net Investment Income [Line Items]
Total investment income	$ 18,147	$ 17,972
Investment securities available-for-sale [Member]
Net Investment Income [Line Items]
Taxable	11,380	12,923
Tax-advantaged	1,419	5,011
Investment securities held-to-maturity [Member]
Net Investment Income [Line Items]
Taxable	2,620	38
Tax-advantaged	$ 2,728	$ 0

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

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 210.9-04.2,3)
-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=27723805&loc=d3e24721-111560

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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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