Mortgage Servicing Assets - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Servicing Asset [Abstract]
Increase (decrease) in value assigned to escrow deposits	1.00%
Increase (decrease) in the fair value of mortgage servicing assets	$ 31
Increase (decrease) in assumed default rate of commercial mortgage loans	1.00%
Increase (decrease) in the fair value of mortgage servicing assets due to change in the assumed default rate of commercial mortgage loans	3
Contractual fee income from servicing commercial mortgage loans	$ 22	$ 22