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, 2012		Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenues	$ 3,400	[1]	$ 3,362	$ 3,423	$ 3,410	$ 3,345	$ 3,420	$ 3,400	$ 3,420	$ 13,595	$ 13,585	$ 13,319
Losses and expenses	3,337	[1]	2,635	2,896	2,731	2,743	3,054	2,854	2,735	11,599	11,386	10,331
Federal and foreign income tax (credit)	(39)		194	123	173	150	68	127	176	451	521	814
NET INCOME	$ 102	[1]	$ 533	$ 404	$ 506	$ 452	$ 298	$ 419	$ 509	$ 1,545	$ 1,678	$ 2,174
Basic earnings per share	$ 0.39	[1]	$ 1.99	$ 1.49	$ 1.85	$ 1.62	$ 1.04	$ 1.43	$ 1.71	$ 5.73	$ 5.80	$ 6.81
Diluted earnings per share	$ 0.38	[1]	$ 1.98	$ 1.48	$ 1.83	$ 1.60	$ 1.04	$ 1.42	$ 1.70	$ 5.69	$ 5.76	$ 6.76
Underwriting ratios
Losses to premiums earned	0.803	[1]	0.538	0.625	0.580	0.593	0.702	0.636	0.620
Expenses to premiums written	0.309	[1]	0.325	0.313	0.322	0.306	0.324	0.313	0.317
Combined loss and expense ratio	1.112	[1]	0.863	0.938	0.902	0.899	1.026	0.949	0.937

[1] In the fourth quarter of 2012, revenues were reduced by net reinsurance reinstatement premium costs of $53 million and losses and expenses included net losses of $829 million related to Storm Sandy. Net income for the quarter was reduced by $573 million or $2.15 per diluted share ($2.16 per share basic) for the after-tax effect of the net costs. Excluding the impact of Storm Sandy, the losses to premiums earned ratio was 50.9%, the expenses to premiums written ratio was 30.4% and the combined ratio was 81.3%.