Shareholders' Equity - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Class of Stock [Line Items]
Remaining under the share repurchase authorization	$ 52,000,000
Authorized repurchase of the Corporation's common stock	1,300,000,000
Dividends paid by domestic property and casualty insurance subsidiaries to parent	2,000,000,000
Maximum [Member]
Class of Stock [Line Items]
Maximum dividend distribution that may be made by the property and casualty insurance subsidiaries to parent company during 2015 without prior regulatory approval	1,900,000,000
Federal Insurance Company [Member]
Class of Stock [Line Items]
Aggregate statutory capital and surplus	14,800,000,000
Risk-based capital requirement	5,000,000,000
P&C Group [Member]
Class of Stock [Line Items]
Aggregate statutory capital and surplus	$ 15,127,000,000	$ 15,024,000,000