Investment Securities (Detail) - The OTTI in the table above was related to two securities with credit-related deterioration evidence	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
ARM Senior Support 2004 [Member]
Credit rating	C	C
Rating agency	Moody's	Moody's
Twelve-month average loss severity	45.96%	38.36%
Twelve-month average default rate	9.31%	3.89%
60 Day or more delinquency rate	25.19%	28.28%
Fixed 5 Year Senior Support 2005 [Member]
Credit rating	CC	CC
Rating agency	Standard & Poor's	Standard & Poor's
Twelve-month average loss severity	76.71%	76.71%
Twelve-month average default rate	2.37%	1.94%
60 Day or more delinquency rate	19.71%	19.23%
ARM Supper Senior Support 2004-2005 [Member]
Credit rating	B	B
Rating agency	Fitch	Fitch
Twelve-month average loss severity	38.06%	51.57%
Twelve-month average default rate	3.67%	2.97%
60 Day or more delinquency rate	13.09%	12.02%