Impairment of Investment Securities - Pooled Trust Preferred Collateralized Debt Obligations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 Bank	Dec. 31, 2014
Impairment Of Investment Securities [Line Items]
Securities available for sale, at fair value	$ 1,269,285	$ 1,309,819
Unrealized Gain (Loss)	17,974	24,420
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Impairment Of Investment Securities [Line Items]
Book Value	41,868
Securities available for sale, at fair value	31,134
Unrealized Gain (Loss)	(10,734)
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL IV [Member]
Impairment Of Investment Securities [Line Items]
Book Value	1,830
Securities available for sale, at fair value	1,324
Unrealized Gain (Loss)	(506)
Debt Instrument, Credit Rating	B1/BB
Number of Banks (in banks)	6
Deferrals and Defaults as a % of Current Collateral	18.05%
Excess Subordination as a % of Current Performing Collateral	56.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL VII [Member]
Impairment Of Investment Securities [Line Items]
Book Value	2,789
Securities available for sale, at fair value	3,147
Unrealized Gain (Loss)	358
Debt Instrument, Credit Rating	Ca/-
Number of Banks (in banks)	14
Deferrals and Defaults as a % of Current Collateral	49.68%
Excess Subordination as a % of Current Performing Collateral	0.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL VIII [Member]
Impairment Of Investment Securities [Line Items]
Book Value	1,984
Securities available for sale, at fair value	1,446
Unrealized Gain (Loss)	(538)
Debt Instrument, Credit Rating	C/C
Number of Banks (in banks)	29
Deferrals and Defaults as a % of Current Collateral	58.01%
Excess Subordination as a % of Current Performing Collateral	0.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL IX [Member]
Impairment Of Investment Securities [Line Items]
Book Value	2,349
Securities available for sale, at fair value	1,597
Unrealized Gain (Loss)	(752)
Debt Instrument, Credit Rating	B3/C
Number of Banks (in banks)	40
Deferrals and Defaults as a % of Current Collateral	28.91%
Excess Subordination as a % of Current Performing Collateral	6.61%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL X [Member]
Impairment Of Investment Securities [Line Items]
Book Value	1,524
Securities available for sale, at fair value	1,669
Unrealized Gain (Loss)	145
Debt Instrument, Credit Rating	Caa1/C
Number of Banks (in banks)	43
Deferrals and Defaults as a % of Current Collateral	31.60%
Excess Subordination as a % of Current Performing Collateral	0.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL XII [Member]
Impairment Of Investment Securities [Line Items]
Book Value	5,559
Securities available for sale, at fair value	3,969
Unrealized Gain (Loss)	(1,590)
Debt Instrument, Credit Rating	B3/C
Number of Banks (in banks)	66
Deferrals and Defaults as a % of Current Collateral	25.23%
Excess Subordination as a % of Current Performing Collateral	0.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL XIII [Member]
Impairment Of Investment Securities [Line Items]
Book Value	12,525
Securities available for sale, at fair value	8,870
Unrealized Gain (Loss)	(3,655)
Debt Instrument, Credit Rating	B3/C
Number of Banks (in banks)	56
Deferrals and Defaults as a % of Current Collateral	25.44%
Excess Subordination as a % of Current Performing Collateral	20.85%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL XIV [Member]
Impairment Of Investment Securities [Line Items]
Book Value	12,876
Securities available for sale, at fair value	8,702
Unrealized Gain (Loss)	(4,174)
Debt Instrument, Credit Rating	Caa1/C
Number of Banks (in banks)	56
Deferrals and Defaults as a % of Current Collateral	25.46%
Excess Subordination as a % of Current Performing Collateral	38.55%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | MMCap I [Member]
Impairment Of Investment Securities [Line Items]
Book Value	432
Securities available for sale, at fair value	410
Unrealized Gain (Loss)	$ (22)
Debt Instrument, Credit Rating	Ca/C
Number of Banks (in banks)	10
Deferrals and Defaults as a % of Current Collateral	56.11%
Excess Subordination as a % of Current Performing Collateral	24.92%