Securities Financing Activities - Summarized Securities Financing Agreements (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	$ 5	$ 4	$ 8
Securities borrowed	1		1
Total offsetting of financial assets	6	4	9
Repurchase agreements
Total offsetting of financial liabilities
Gross Amount Presented In Balance Sheet [Member]
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	311	271	292
Securities borrowed	35		19
Total offsetting of financial assets	346	271	311
Repurchase agreements	494	228	394
Total offsetting of financial liabilities	494	228	394
Netting Adjustments [Member]
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	(195)	(95)	(137)
Securities borrowed
Total offsetting of financial assets	(195)	(95)	(137)
Repurchase agreements	(195)	(95)	(137)
Total offsetting of financial liabilities	(195)	(95)	(137)
Collateral [Member]
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	(111)	(172)	(147)
Securities borrowed	(34)		(18)
Total offsetting of financial assets	(145)	(172)	(165)
Repurchase agreements	(299)	(133)	(257)
Total offsetting of financial liabilities	$ (299)	$ (133)	$ (257)