Impairment of Investment Securities - Pooled Trust Preferred Collateralized Debt Obligations (Detail) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($) Bank	Dec. 31, 2016 USD ($)
Impairment Of Investment Securities [Line Items]
Amortized Cost	$ 740,846	$ 790,082
Estimated Fair Value of Available for Sale	731,358	$ 778,612
Mezzanine [Member]
Impairment Of Investment Securities [Line Items]
Amortized Cost	27,499
Estimated Fair Value of Available for Sale	23,646
Unrealized Gain (Loss)	(3,853)
Mezzanine [Member] \| Pre TSL IV [Member]
Impairment Of Investment Securities [Line Items]
Amortized Cost	1,817
Estimated Fair Value of Available for Sale	1,405
Unrealized Gain (Loss)	$ (412)
Debt Instrument, Credit Rating	Ba1/BB
Number of Banks \| Bank	6
Deferrals and Defaults as a % of Current Collateral	18.05%
Excess Subordination as a % of Current Performing Collateral	72.13%
Mezzanine [Member] \| Pre TSL VIII [Member]
Impairment Of Investment Securities [Line Items]
Amortized Cost	$ 2,043
Estimated Fair Value of Available for Sale	2,228
Unrealized Gain (Loss)	$ 185
Debt Instrument, Credit Rating	C/C
Number of Banks \| Bank	26
Deferrals and Defaults as a % of Current Collateral	38.52%
Excess Subordination as a % of Current Performing Collateral	0.00%
Mezzanine [Member] \| Pre TSL IX [Member]
Impairment Of Investment Securities [Line Items]
Amortized Cost	$ 2,448
Estimated Fair Value of Available for Sale	2,052
Unrealized Gain (Loss)	$ (396)
Debt Instrument, Credit Rating	B1/C
Number of Banks \| Bank	37
Deferrals and Defaults as a % of Current Collateral	27.83%
Excess Subordination as a % of Current Performing Collateral	19.46%
Mezzanine [Member] \| Pre TSL X [Member]
Impairment Of Investment Securities [Line Items]
Amortized Cost	$ 1,863
Estimated Fair Value of Available for Sale	2,125
Unrealized Gain (Loss)	$ 262
Debt Instrument, Credit Rating	Caa1/C
Number of Banks \| Bank	41
Deferrals and Defaults as a % of Current Collateral	27.93%
Excess Subordination as a % of Current Performing Collateral	1.62%
Mezzanine [Member] \| Pre TSL XII [Member]
Impairment Of Investment Securities [Line Items]
Amortized Cost	$ 6,097
Estimated Fair Value of Available for Sale	5,209
Unrealized Gain (Loss)	$ (888)
Debt Instrument, Credit Rating	B3/C
Number of Banks \| Bank	63
Deferrals and Defaults as a % of Current Collateral	23.35%
Excess Subordination as a % of Current Performing Collateral	0.00%
Mezzanine [Member] \| Pre TSL XIV [Member]
Impairment Of Investment Securities [Line Items]
Amortized Cost	$ 13,136
Estimated Fair Value of Available for Sale	10,453
Unrealized Gain (Loss)	$ (2,683)
Debt Instrument, Credit Rating	Ba2/CCC
Number of Banks \| Bank	49
Deferrals and Defaults as a % of Current Collateral	12.95%
Excess Subordination as a % of Current Performing Collateral	39.26%
Mezzanine [Member] \| MMCap I [Member]
Impairment Of Investment Securities [Line Items]
Amortized Cost	$ 95
Estimated Fair Value of Available for Sale	174
Unrealized Gain (Loss)	$ 79
Debt Instrument, Credit Rating	Ca/C
Number of Banks \| Bank	7
Deferrals and Defaults as a % of Current Collateral	69.35%
Excess Subordination as a % of Current Performing Collateral	69.99%

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Excess subordination as a percentage of current performing collateral.

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Impairment Of Investment Securities [Line Items]

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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)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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Amount before tax of unrealized gain (loss) in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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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)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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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
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21475-112644

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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