Changes of Unamortized Discount and Designated Credit Reserves on Available for Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified			3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 Residential	Jun. 30, 2012 Residential	Jun. 30, 2012 Commercial	Jun. 30, 2012 Commercial	Jun. 30, 2012 CDO	Jun. 30, 2012 CDO
Credit Reserve
Beginning Balance	$ 261,714	$ 295,990	$ 235,618	$ 242,261	$ 33,668	$ 43,012	$ 7,244	$ 10,717
Amortization of net discount
Realized credit losses			(11,786)	(25,908)	(624)	(7,897)		(3,500)
Acquisitions			4,870	11,552
Sales, calls, other			(10,598)	(16,156)
Impairments			303	524		111
Transfers to (release of) credit reserves			3,019	9,153	(31)	(2,213)	31	58
Ending Balance	261,714	295,990	221,426	221,426	33,013	33,013	7,275	7,275
Unamortized Discount Net
Beginning Balance	230,356	239,387	238,533	235,833	5,651	3,554
Amortization of net discount			(8,198)	(16,398)	(150)	(235)	31	58
Realized credit losses
Acquisitions			8,392	43,998
Sales, calls, other			(10,884)	(29,456)
Impairments
Transfers to (release of) credit reserves			(3,019)	(9,153)	31	2,213	(31)	(58)
Ending Balance	$ 230,356	$ 239,387	$ 224,824	$ 224,824	$ 5,532	$ 5,532