Schedule I-Parent Company Financial Information (Notes to Condensed Financial Statements) - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Condensed Financial Statements, Captions [Line Items]
Cash dividends received from subsidiaries	$ 1,200,000,000	$ 1,100,000,000	$ 747,000,000
Interest-bearing rate minimum	1.40%
Interest-bearing rate maximum	6.65%
Interest expense on subsidiary notes payable	1,000,000	1,000,000	1,000,000
Aggregate statutory capital and surplus in our state regulated subsidiaries	5,100,000,000	4,700,000,000
Dividends that were paid to the parent company without prior approval by state regulatory authorities	860,000,000
Dividends that may be paid to the parent company without prior approval by state regulatory authorities in 2013	$ 911,000,000