Fair Value of the Servicing Assets (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Sep. 30, 2013 Fixed Rate Residential Mortgage	Dec. 31, 2012 Fixed Rate Residential Mortgage	Sep. 30, 2012 Fixed Rate Residential Mortgage	Dec. 31, 2011 Fixed Rate Residential Mortgage	Sep. 30, 2013 Adjustable Rate Residential Mortgage	Dec. 31, 2012 Adjustable Rate Residential Mortgage	Sep. 30, 2012 Adjustable Rate Residential Mortgage	Dec. 31, 2011 Adjustable Rate Residential Mortgage	Sep. 30, 2013 Fixed Rate Automobile Loans
Servicing Assets at Fair Value
Fair value at beginning of period	$ 6	$ 876	$ 664	$ 645	$ 649	$ 39	$ 33	$ 34	$ 32	$ 4
Fair value at end of period	$ 6	$ 876	$ 664	$ 645	$ 649	$ 39	$ 33	$ 34	$ 32	$ 4