Goodwill and Other Intangible Assets - Schedule of Gross Carrying Amount and Accumulated Amortization of Other Intangible Assets (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 111,615	$ 103,490
Accumulated Amortization and Impairment	(67,877)	(64,796)
Net Carrying Amount	43,738us-gaap_FiniteLivedIntangibleAssetsNet	38,694us-gaap_FiniteLivedIntangibleAssetsNet	24,120us-gaap_FiniteLivedIntangibleAssetsNet
Core Deposit Intangible [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	60,103	57,149
Accumulated Amortization and Impairment	(38,833)	(36,950)
Net Carrying Amount	21,270	20,199
Customer Business Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	30,552	27,942
Accumulated Amortization and Impairment	(21,915)	(21,438)
Net Carrying Amount	8,637	6,504
Trust Customer Relationship Intangible [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	16,547	13,986
Accumulated Amortization and Impairment	(3,853)	(3,232)
Net Carrying Amount	12,694	10,754
Customer Loan Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	4,413	4,413
Accumulated Amortization and Impairment	(3,276)	(3,176)
Net Carrying Amount	$ 1,137	$ 1,237

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

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

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

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