Impairment of Investment Securities - Pooled Trust Preferred Collateralized Debt Obligations (Detail) (USD $) In Thousands, unless otherwise specified																9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Senior [Member] Pre TSL I [Member]	Sep. 30, 2012 Senior [Member] MMCap I [Member]	Sep. 30, 2012 Mezzanine [Member] Pre TSL IV [Member]	Sep. 30, 2012 Mezzanine [Member] Pre TSL V [Member]	Sep. 30, 2012 Mezzanine [Member] Pre TSL VII [Member]	Sep. 30, 2012 Mezzanine [Member] Pre TSL VIII [Member]	Sep. 30, 2012 Mezzanine [Member] Pre TSL IX [Member]	Sep. 30, 2012 Mezzanine [Member] Pre TSL X [Member]	Sep. 30, 2012 Mezzanine [Member] Pre TSL XII [Member]	Sep. 30, 2012 Mezzanine [Member] Pre TSL XIII [Member]	Sep. 30, 2012 Mezzanine [Member] Pre TSL XIV [Member]	Sep. 30, 2012 Mezzanine [Member] MMCap I [Member]	Sep. 30, 2012 Mezzanine [Member] MM Comm IX [Member]	Sep. 30, 2012 Preferred Collateralized Debt Obligations Total [Member] Moody's, C Rating [Member] Mezzanine [Member] Pre TSL V [Member] Bank	Sep. 30, 2012 Preferred Collateralized Debt Obligations Total [Member] Fitch, A Rating [Member] Moody's, Aa3 Rating [Member] Senior [Member] Pre TSL I [Member] Bank	Sep. 30, 2012 Preferred Collateralized Debt Obligations Total [Member] Fitch, A Rating [Member] Moody's, A3 Rating [Member] Senior [Member] MMCap I [Member] Bank	Sep. 30, 2012 Preferred Collateralized Debt Obligations Total [Member] Fitch, CCC Rating [Member] Moody's, Caa2 Rating [Member] Mezzanine [Member] Pre TSL IV [Member] Bank	Sep. 30, 2012 Preferred Collateralized Debt Obligations Total [Member] Fitch, C Rating [Member] Moody's, C Rating [Member] Mezzanine [Member] Pre TSL VIII [Member] Bank	Sep. 30, 2012 Preferred Collateralized Debt Obligations Total [Member] Fitch, C Rating [Member] Moody's, C Rating [Member] Mezzanine [Member] Pre TSL X [Member] Bank	Sep. 30, 2012 Preferred Collateralized Debt Obligations Total [Member] Fitch, C Rating [Member] Moody's, Ca Rating [Member] Mezzanine [Member] Pre TSL VII [Member] Bank	Sep. 30, 2012 Preferred Collateralized Debt Obligations Total [Member] Fitch, C Rating [Member] Moody's, Ca Rating [Member] Mezzanine [Member] Pre TSL IX [Member] Bank	Sep. 30, 2012 Preferred Collateralized Debt Obligations Total [Member] Fitch, C Rating [Member] Moody's, Ca Rating [Member] Mezzanine [Member] Pre TSL XII [Member] Bank	Sep. 30, 2012 Preferred Collateralized Debt Obligations Total [Member] Fitch, C Rating [Member] Moody's, Ca Rating [Member] Mezzanine [Member] Pre TSL XIII [Member] Bank	Sep. 30, 2012 Preferred Collateralized Debt Obligations Total [Member] Fitch, C Rating [Member] Moody's, Ca Rating [Member] Mezzanine [Member] Pre TSL XIV [Member] Bank	Sep. 30, 2012 Preferred Collateralized Debt Obligations Total [Member] Fitch, C Rating [Member] Moody's, Ca Rating [Member] Mezzanine [Member] MMCap I [Member] Bank	Sep. 30, 2012 Preferred Collateralized Debt Obligations Total [Member] Fitch, CC Rating [Member] Moody's, Ca Rating [Member] Mezzanine [Member] MM Comm IX [Member] Bank
Impairment Of Investment Securities [Line Items]
Book value	$ 52,696		$ 1,009	$ 1,898	$ 1,830	$ 53	$ 4,114	$ 1,794	$ 2,259	$ 1,447	$ 5,516	$ 12,418	$ 13,069	$ 849	$ 6,440
Fair Value	1,130,822	1,142,776	993	1,841	645	55	3,419	1,016	811	1,118	2,696	4,714	4,570	455	718
Unrealized Gain (Loss)	$ (29,645)		$ (16)	$ (57)	$ (1,185)	$ 2	$ (695)	$ (778)	$ (1,448)	$ (329)	$ (2,820)	$ (7,704)	$ (8,499)	$ (394)	$ (5,722)
Number of Banks																3	17	17	6	34	51	17	47	73	63	61	17	30
Deferrals and Defaults as a % of Current Collateral																100.00%	38.98%	48.60%	27.07%	48.43%	38.24%	45.50%	25.88%	34.04%	37.50%	40.08%	48.60%	39.29%
Excess Subordination as a % of Current Performing Collateral																0.00%	213.61%	565.56%	95.96%	0.00%	0.00%	0.00%	9.64%	0.00%	13.84%	32.10%	14.50%	0.00%