Investment Securities - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2012 Y CreditScore	Sep. 30, 2011	Jun. 30, 2012	Sep. 30, 2012 Y CreditScore	Sep. 30, 2011	Dec. 31, 2011 Y
Federal statutory rate	35.00%	35.00%		35.00%	35.00%
Minimum weighted average credit score of prime securities	725			725
Maximum loan-to-value of prime securities	80.00%			80.00%
Available-for-sale agency mortgage-backed securities transferred to held-to-maturity			$ 11,700,000,000
Weighted-average maturity of the available-for-sale investment securities (in years)	4.0			4.0		5.2
Weighted-average yields of available-for-sale investment securities	3.04%			3.04%		3.19%
Weighted-average maturity of the held-to-maturity investment securities (in years)	3.1			3.1		3.9
Weighted-average yields of held-to-maturity investment securities	2.06%			2.06%		2.21%
Securities pledged	16,500,000,000			16,500,000,000		20,700,000,000
Securities pledged as collateral where counterparty has right to repledge or resell	3,400,000,000			3,400,000,000		7,000,000,000
Available-for-sale and held-to-maturity unrealized losses on residential non-agency mortgage-backed securities				$ 97,000,000