Goodwill and Other Acquired Intangible Assets - Schedule of Definite-Lived Intangible Assets (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 57,665	$ 46,754
Accumulated Amortization	(37,304)	(31,415)
Net Carrying Amount	20,361us-gaap_FiniteLivedIntangibleAssetsNet	15,339us-gaap_FiniteLivedIntangibleAssetsNet
Core Deposit Intangibles [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	55,949	45,406
Accumulated Amortization	(36,354)	(30,784)
Net Carrying Amount	19,595	14,622
Customer Relationship Intangible Asset [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,348	1,348
Accumulated Amortization	(822)	(631)
Net Carrying Amount	526	717
Noncompete Agreements [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	163
Accumulated Amortization	(82)
Net Carrying Amount	81
Other Intangible Assets [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	205
Accumulated Amortization	(46)
Net Carrying Amount	$ 159

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Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275

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- Definition

Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

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Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

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