Securities Financing Activities - Summarized Securities Financing Agreements (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	$ 8	$ 4	$ 5
Securities borrowed
Total offsetting of financial assets	8	4	5
Repurchase agreements
Total offsetting of financial liabilities
Gross Amount Presented in Balance Sheet [Member]
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	516	271	294
Securities borrowed	1		3
Total offsetting of financial assets	517	271	297
Repurchase agreements	464	228	398
Total offsetting of financial liabilities	464	228	398
Netting Adjustments [Member]
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	(176)	(95)	(171)
Securities borrowed
Total offsetting of financial assets	(176)	(95)	(171)
Repurchase agreements	(176)	(95)	(171)
Total offsetting of financial liabilities	(176)	(95)	(171)
Collateral [Member]
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	(332)	(172)	(118)
Securities borrowed	(1)		(3)
Total offsetting of financial assets	(333)	(172)	(121)
Repurchase agreements	(288)	(133)	(227)
Total offsetting of financial liabilities	$ (288)	$ (133)	$ (227)