Investment Securities - Summary of Interest Income Earned on Investment Securities (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Net Investment Income [Line Items]
Total interest income from available-for-sale securities	$ 13,007	$ 12,018	$ 25,647	$ 24,817
Total interest income from held-to-maturity securities	5,323	4,743	10,830	10,091
Total investment income	18,330	16,761	36,477	34,908
Investment securities available-for-sale [Member]
Net Investment Income [Line Items]
Taxable	12,420	10,827	24,346	22,207
Tax-advantaged	587	1,191	1,301	2,610
Investment securities held-to-maturity [Member]
Net Investment Income [Line Items]
Taxable	3,203	2,215	6,480	4,835
Tax-advantaged	$ 2,120	$ 2,528	$ 4,350	$ 5,256

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.

+ 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
-Glossary Available-for-Sale Securities
-URI http://asc.fasb.org/extlink&oid=6505594

Reference 3: 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 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=27723805&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=65884222&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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