Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (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
Basic earnings per share:
Net Income	$ 102	[1]	$ 533	$ 404	$ 506	$ 452	$ 298	$ 419	$ 509	$ 1,545	$ 1,678	$ 2,174
Weighted average shares outstanding										269.5	289.3	319.2
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:
Net Income	$ 102	[1]	$ 533	$ 404	$ 506	$ 452	$ 298	$ 419	$ 509	$ 1,545	$ 1,678	$ 2,174
Weighted average shares outstanding										269.5	289.3	319.2
Additional shares from assumed issuance of shares under stock-based compensation awards										1.9	2.1	2.4
Weighted average shares and potential shares assumed outstanding for computing diluted earnings per share										271.4	291.4	321.6
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

[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%.