Short-Term Debt - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Short-term Debt [Line Items]
Residential loan pledged as collateral	$ 1,200		$ 1,200
Securities pledged as collateral	455		455
Average balance of short-term debt	1,050	474	954	418
Accrued interest payable on short-term debt	$ 1		$ 1		$ 1
Credit Agreement
Short-term Debt [Line Items]
Number of bank counterparties	11		11