Impairment of Investment Securities - Pooled Trust Preferred Collateralized Debt Obligations (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2015 USD ($) Bank	Dec. 31, 2014 USD ($)
Impairment Of Investment Securities [Line Items]
Securities available for sale, at fair value	$ 1,050,733	$ 1,309,819
Unrealized Gain (Loss)	12,186	$ 24,420
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Impairment Of Investment Securities [Line Items]
Book Value	42,000
Securities available for sale, at fair value	35,779
Unrealized Gain (Loss)	(6,221)
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	1,337
Unrealized Gain (Loss)	$ (493)
Debt Instrument, Credit Rating	B1/BB
Number of Banks (in banks) | Bank	6
Deferrals and Defaults as a % of Current Collateral	18.05%
Excess Subordination as a % of Current Performing Collateral	57.96%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL VII [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 2,905
Securities available for sale, at fair value	3,494
Unrealized Gain (Loss)	$ 589
Debt Instrument, Credit Rating	Ca/-
Number of Banks (in banks) | Bank	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	$ 2,000
Securities available for sale, at fair value	1,865
Unrealized Gain (Loss)	$ (135)
Debt Instrument, Credit Rating	C/C
Number of Banks (in banks) | Bank	29
Deferrals and Defaults as a % of Current Collateral	56.74%
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	$ 2,367
Securities available for sale, at fair value	1,811
Unrealized Gain (Loss)	$ (556)
Debt Instrument, Credit Rating	B1/C
Number of Banks (in banks) | Bank	38
Deferrals and Defaults as a % of Current Collateral	29.80%
Excess Subordination as a % of Current Performing Collateral	10.04%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL X [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 1,583
Securities available for sale, at fair value	1,921
Unrealized Gain (Loss)	$ 338
Debt Instrument, Credit Rating	Caa1/C
Number of Banks (in banks) | Bank	43
Deferrals and Defaults as a % of Current Collateral	30.66%
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	$ 5,643
Securities available for sale, at fair value	4,598
Unrealized Gain (Loss)	$ (1,045)
Debt Instrument, Credit Rating	B3/C
Number of Banks (in banks) | Bank	66
Deferrals and Defaults as a % of Current Collateral	23.07%
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	$ 12,646
Securities available for sale, at fair value	10,704
Unrealized Gain (Loss)	$ (1,942)
Debt Instrument, Credit Rating	B3/C
Number of Banks (in banks) | Bank	56
Deferrals and Defaults as a % of Current Collateral	12.05%
Excess Subordination as a % of Current Performing Collateral	26.32%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL XIV [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 12,818
Securities available for sale, at fair value	9,752
Unrealized Gain (Loss)	$ (3,066)
Debt Instrument, Credit Rating	Caa1/CC
Number of Banks (in banks) | Bank	56
Deferrals and Defaults as a % of Current Collateral	19.72%
Excess Subordination as a % of Current Performing Collateral	43.57%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | MMCap I [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 208
Securities available for sale, at fair value	297
Unrealized Gain (Loss)	$ 89
Debt Instrument, Credit Rating	Ca/C
Number of Banks (in banks) | Bank	8
Deferrals and Defaults as a % of Current Collateral	58.11%
Excess Subordination as a % of Current Performing Collateral	84.08%