Impairment of Investment Securities - Pooled Trust Preferred Collateralized Debt Obligations (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2015 USD ($) Bank	Dec. 31, 2014 USD ($)
Impairment Of Investment Securities [Line Items]
Securities available for sale, at fair value	$ 1,089,725	$ 1,309,819
Unrealized Gain (Loss)	16,419	$ 24,420
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Impairment Of Investment Securities [Line Items]
Book Value	41,745
Securities available for sale, at fair value	35,521
Unrealized Gain (Loss)	(6,224)
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	1,331
Unrealized Gain (Loss)	$ (499)
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	57.14%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL VII [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 2,847
Securities available for sale, at fair value	3,478
Unrealized Gain (Loss)	$ 631
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	$ 1,982
Securities available for sale, at fair value	1,791
Unrealized Gain (Loss)	$ (191)
Debt Instrument, Credit Rating	C/C
Number of Banks (in banks) | Bank	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	$ 2,358
Securities available for sale, at fair value	1,906
Unrealized Gain (Loss)	$ (452)
Debt Instrument, Credit Rating	B1/C
Number of Banks (in banks) | Bank	38
Deferrals and Defaults as a % of Current Collateral	28.14%
Excess Subordination as a % of Current Performing Collateral	10.73%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL X [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 1,553
Securities available for sale, at fair value	1,889
Unrealized Gain (Loss)	$ 336
Debt Instrument, Credit Rating	Caa1/C
Number of Banks (in banks) | Bank	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	$ 5,599
Securities available for sale, at fair value	4,502
Unrealized Gain (Loss)	$ (1,097)
Debt Instrument, Credit Rating	B3/C
Number of Banks (in banks) | Bank	66
Deferrals and Defaults as a % of Current Collateral	23.76%
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	$ 12,585
Securities available for sale, at fair value	10,638
Unrealized Gain (Loss)	$ (1,947)
Debt Instrument, Credit Rating	B3/C
Number of Banks (in banks) | Bank	56
Deferrals and Defaults as a % of Current Collateral	25.64%
Excess Subordination as a % of Current Performing Collateral	37.87%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL XIV [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 12,784
Securities available for sale, at fair value	9,691
Unrealized Gain (Loss)	$ (3,093)
Debt Instrument, Credit Rating	Caa1/CC
Number of Banks (in banks) | Bank	56
Deferrals and Defaults as a % of Current Collateral	23.17%
Excess Subordination as a % of Current Performing Collateral	47.33%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | MMCap I [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 207
Securities available for sale, at fair value	295
Unrealized Gain (Loss)	$ 88
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	84.18%