Consolidated Statements Of Operations and Comprehensive Income (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenues
Net premiums written	$ 16,075	$ 15,372	$ 13,708
Change in unearned premiums	(398)	15	(204)
Net premiums earned	15,677	15,387	13,504
Net investment income	2,181	2,242	2,070
Net realized gains (losses):
Other-than-temporary impairment (OTTI) losses gross	(38)	(65)	(128)
Portion of OTTI losses recognized in other comprehensive income (OCI)	1	15	69
Net OTTI losses recognized in income	(37)	(50)	(59)
Net realized gains (losses) excluding OTTI losses	115	(745)	491
Total net realized gains (losses)	78	(795)	432
Total revenues	17,936	16,834	16,006
Expenses
Losses and loss expenses	9,653	9,520	7,579
Policy benefits	521	401	357
Policy acquisition costs	2,446	2,472	2,345
Administrative expenses	2,096	2,068	1,873
Interest expense	250	250	224
Other (income) expense	(6)	81	(10)
Total expenses	14,960	14,792	12,368
Income before income tax	2,976	2,042	3,638
Income tax expense	270	502	553
Net income (loss)	2,706	1,540	3,085
Other comprehensive income (loss)
Unrealized appreciation	1,350	646	1,526
Reclassification adjustment for net realized gains included in net income	(234)	(173)	(632)
Other comprehensive income (loss)	1,116	473	894
Change in:
Cumulative translation adjustment	116	(5)	(7)
Pension liability	(35)	8	11
Other comprehensive income, before income tax	1,197	476	898
Income tax expense related to OCI items	(221)	(159)	(127)
Other comprehensive income (loss)	976	317	771
Comprehensive income	$ 3,682	$ 1,857	$ 3,856
Earnings per share
Basic earnings per share (US$ per share)	$ 7.96	$ 4.55	$ 9.08
Diluted earnings per share (US$ per share)	$ 7.89	$ 4.52	$ 9.04