Impairment of Investment Securities - Pooled Trust Preferred Collateralized Debt Obligations (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Impairment Of Investment Securities [Line Items]
Amortized Cost	$ 1,317,318	$ 1,350,466
Estimated Fair Value of Available for Sale	1,309,819us-gaap_AvailableForSaleSecurities	1,318,365us-gaap_AvailableForSaleSecurities
Mezzanine [Member]
Impairment Of Investment Securities [Line Items]
Amortized Cost	41,926
Estimated Fair Value of Available for Sale	28,999
Unrealized Gain (Loss)	(12,927)
Mezzanine [Member] \| Pre TSL IV [Member]
Impairment Of Investment Securities [Line Items]
Amortized Cost	1,830
Estimated Fair Value of Available for Sale	1,326
Unrealized Gain (Loss)	(504)
Debt Instrument, Credit Rating	B1/BB
Number of Banks	6
Deferrals and Defaults as a % of Current Collateral	18.05%
Excess Subordination as a % of Current Performing Collateral	54.84%
Mezzanine [Member] \| Pre TSL V [Member]
Impairment Of Investment Securities [Line Items]
Amortized Cost	60
Estimated Fair Value of Available for Sale	130
Unrealized Gain (Loss)	70
Debt Instrument, Credit Rating	C/-
Number of Banks	3
Deferrals and Defaults as a % of Current Collateral	100.00%
Excess Subordination as a % of Current Performing Collateral	0.00%
Mezzanine [Member] \| Pre TSL VII [Member]
Impairment Of Investment Securities [Line Items]
Amortized Cost	2,779
Estimated Fair Value of Available for Sale	2,987
Unrealized Gain (Loss)	208
Debt Instrument, Credit Rating	Ca/-
Number of Banks	14
Deferrals and Defaults as a % of Current Collateral	49.68%
Excess Subordination as a % of Current Performing Collateral	0.00%
Mezzanine [Member] \| Pre TSL VIII [Member]
Impairment Of Investment Securities [Line Items]
Amortized Cost	1,970
Estimated Fair Value of Available for Sale	1,305
Unrealized Gain (Loss)	(665)
Debt Instrument, Credit Rating	C/C
Number of Banks	29
Deferrals and Defaults as a % of Current Collateral	59.12%
Excess Subordination as a % of Current Performing Collateral	0.00%
Mezzanine [Member] \| Pre TSL IX [Member]
Impairment Of Investment Securities [Line Items]
Amortized Cost	2,340
Estimated Fair Value of Available for Sale	1,442
Unrealized Gain (Loss)	(898)
Debt Instrument, Credit Rating	B3/C
Number of Banks	39
Deferrals and Defaults as a % of Current Collateral	28.91%
Excess Subordination as a % of Current Performing Collateral	6.73%
Mezzanine [Member] \| Pre TSL X [Member]
Impairment Of Investment Securities [Line Items]
Amortized Cost	1,495
Estimated Fair Value of Available for Sale	1,526
Unrealized Gain (Loss)	31
Debt Instrument, Credit Rating	Caa1/C
Number of Banks	44
Deferrals and Defaults as a % of Current Collateral	31.03%
Excess Subordination as a % of Current Performing Collateral	0.00%
Mezzanine [Member] \| Pre TSL XII [Member]
Impairment Of Investment Securities [Line Items]
Amortized Cost	5,523
Estimated Fair Value of Available for Sale	3,361
Unrealized Gain (Loss)	(2,162)
Debt Instrument, Credit Rating	B3/C
Number of Banks	66
Deferrals and Defaults as a % of Current Collateral	26.96%
Excess Subordination as a % of Current Performing Collateral	0.00%
Mezzanine [Member] \| Pre TSL XIII [Member]
Impairment Of Investment Securities [Line Items]
Amortized Cost	12,464
Estimated Fair Value of Available for Sale	8,455
Unrealized Gain (Loss)	(4,009)
Debt Instrument, Credit Rating	Caa1/C
Number of Banks	57
Deferrals and Defaults as a % of Current Collateral	19.26%
Excess Subordination as a % of Current Performing Collateral	30.97%
Mezzanine [Member] \| Pre TSL XIV [Member]
Impairment Of Investment Securities [Line Items]
Amortized Cost	13,035
Estimated Fair Value of Available for Sale	8,081
Unrealized Gain (Loss)	(4,954)
Debt Instrument, Credit Rating	Caa1/C
Number of Banks	56
Deferrals and Defaults as a % of Current Collateral	24.54%
Excess Subordination as a % of Current Performing Collateral	36.53%
Mezzanine [Member] \| MMCap I [Member]
Impairment Of Investment Securities [Line Items]
Amortized Cost	430
Estimated Fair Value of Available for Sale	386
Unrealized Gain (Loss)	$ (44)
Debt Instrument, Credit Rating	Ca/C
Number of Banks	11
Deferrals and Defaults as a % of Current Collateral	59.52%
Excess Subordination as a % of Current Performing Collateral	20.89%
Maximum [Member]
Impairment Of Investment Securities [Line Items]
Excess Subordination as a % of Current Performing Collateral	55.00%

Deferrals and defaults as a percentage of current collateral.

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Excess subordination as a percentage of current performing collateral.

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- Definition

Pooled trust preferred collateralized debt obligations in number of banks.

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- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

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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 gain (loss) on investments in debt and equity securities classified as 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 9

-Subparagraph (d)

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- Definition

Description of the credit rating indicative of likelihood of repayment of the debt instrument given by a credit rating agency (for example, Standard and Poor's, Moody's, Fitch).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 3

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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