Mortgage Servicing Assets - Changes in Carrying Amount of Mortgage Servicing Assets (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Servicing Assets at Amortized Value [Line Items]
Balance at beginning of period	$ 173	$ 196
Servicing retained from loan sales	47	23
Purchases	44	13
Amortization	(60)	(59)
Balance at end of period	204	173
Fair value at end of period	$ 238	$ 245