Short-Term Debt - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Short-term Debt [Line Items]
Residential loan pledged as collateral	$ 519		$ 519
Securities pledged as collateral	500		500
Average balance of short-term debt	1,150	571	1,020	469
Accrued interest payable on short-term debt	$ 1		$ 1		$ 1
Credit Agreement
Short-term Debt [Line Items]
Number of bank counterparties	13		13