Mortgage Servicing Assets - Changes in Carrying Amount of Mortgage Servicing Assets (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Sep. 30, 2013	Sep. 30, 2012
Servicing Asset [Abstract]
Balance at beginning of period			$ 204	$ 173
Servicing retained from loan sales			31	30
Purchases	118	118	144	31
Amortization			(48)	(46)
Balance at end of period	331		331	188
Fair value at end of period	$ 388		$ 388	$ 237