Quarterly Financial Data - Summary of Quarterly Financial Data (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 3,476	$ 3,403	$ 3,551	$ 3,517	$ 3,400	$ 3,362	$ 3,423	$ 3,410	$ 13,947	$ 13,595	$ 13,585
Losses and expenses	2,690	2,664	2,755	2,601	3,337	2,635	2,896	2,731
Federal and foreign income tax (credit)	217	198	217	260	(39)	194	123	173	892	451	521
NET INCOME	$ 569	$ 541	$ 579	$ 656	$ 102	$ 533	$ 404	$ 506	$ 2,345	$ 1,545	$ 1,678
Basic earnings per share	$ 2.25	$ 2.11	$ 2.22	$ 2.49	$ 0.39	$ 1.99	$ 1.49	$ 1.85	$ 9.08	$ 5.73	$ 5.80
Diluted earnings per share	$ 2.24	$ 2.10	$ 2.21	$ 2.48	$ 0.38	$ 1.98	$ 1.48	$ 1.83	$ 9.04	$ 5.69	$ 5.76
Underwriting ratios
Losses to premiums earned	0.547	0.530	0.567	0.523	0.803	0.538	0.625	0.580
Expenses to premiums written	0.308	0.327	0.321	0.323	0.309	0.325	0.313	0.322
Combined loss and expense ratio	0.855	0.857	0.888	0.846	1.112	0.863	0.938	0.902