Investment Securities - Additional Information (Detail) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Maturity of contract	less than one year
Approximate year of maturity	30 years
Gross Unrealized Losses	$ 24,420,000	$ 47,666,000
Amortized Cost	1,317,318,000	1,350,466,000
Net securities gains (losses)	550,000us-gaap_GainLossOnSaleOfSecuritiesNet	(1,158,000)us-gaap_GainLossOnSaleOfSecuritiesNet	192,000us-gaap_GainLossOnSaleOfSecuritiesNet
Available-for-sale securities pledged as collateral	563,200,000	594,900,000
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Losses	500,000	1,300,000
Amortized Cost	6,600,000
Net securities gains (losses)	200,000
Pooled Trust Preferred Collateralized Debt Obligations [Member] \| Non-bank subsidiaries [Member]
Schedule of Available-for-sale Securities [Line Items]
Sale of available securities	200,000
Single issuer trust preferred securities [Member] \| Non-bank subsidiaries [Member]
Schedule of Available-for-sale Securities [Line Items]
Sale of available securities	$ 5,100,000

Contractual Maturity Of Securities

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- Definition

Sales Of Available For Sale Securities

+ References

No definition available.

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- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

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- Definition

Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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- Definition

The fair value of available-for-sale securities that serve as collateral for borrowings.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5086-111524

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- Definition

Description of how prepayments are considered in determination of contractual cash flows and cash flows expected to be collected for the loan or debt security. This relates to loans and debt securities for which at acquisition it was probable that the investor would be unable to collect all contractually required payments receivable or all cash flows expected at acquisition.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15029-111544

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-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 15

-Paragraph 2

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- Definition

The net gain (loss) realized from the sale, exchange, redemption, or retirement of securities, not separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

+ 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 210.9-04.13(h))

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 13

-Subparagraph h

-Article 9

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.3)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

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-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 3

-Article 7

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