Changes in the Servicing Asset Related to Residential Mortgage Loans (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Servicing Assets at Fair Value [Line Items]
Carrying amount as of the beginning of the period	$ 1,239	$ 1,138
Servicing obligations that result from the transfer of residential mortgage loans	254	155
Amortization	(134)	(87)
Carrying amount before valuation allowance	1,359	1,206
Valuation allowance for servicing assets:
Beginning balance	(558)	(316)
Servicing impairment	(122)	(228)
Ending balance	(680)	(544)
Carrying amount as of the end of the period	$ 679	$ 662