Shareholders' Equity - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Jan. 31, 2013 Subsequent Event [Member]
Remaining under the share repurchase authorization	$ 329,000,000
Authorized repurchase of the Corporation's common stock		1,300,000,000
Dividends paid by domestic property and casualty insurance subsidiaries to parent	1,800,000,000
Maximum dividend distribution that may be made by the property and casualty insurance subsidiaries to parent company during 2013 without prior regulatory approval	$ 1,600,000,000