Consolidated Statements Of Operations and Comprehensive Income (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Revenues
Net premiums written	$ 4,716	$ 4,343	$ 12,418	$ 11,742
Change in unearned premiums	(51)	147	(589)	(186)
Net premiums earned	4,665	4,490	11,829	11,556
Net investment income	533	564	1,614	1,677
Net realized gains (losses):
Other-than-temporary impairment (OTTI) losses gross	(10)	(31)	(30)	(45)
Portion of OTTI losses recognized in other comprehensive income (OCI)		11		13
Net OTTI losses recognized in income	(10)	(20)	(30)	(32)
Net realized gains (losses) excluding OTTI losses	(50)	(740)	(164)	(846)
Total net realized gains (losses)	(60)	(760)	(194)	(878)
Total revenues	5,138	4,294	13,249	12,355
Expenses
Losses and loss expenses	3,047	2,745	6,970	7,234
Policy benefits	130	83	379	282
Policy acquisition costs	609	669	1,810	1,840
Administrative expenses	519	520	1,543	1,537
Interest expense	63	62	187	187
Other (income) expense	(17)	89	14	88
Total expenses	4,351	4,168	10,903	11,168
Income before income tax	787	126	2,346	1,187
Income tax expense	147	165	405	382
Net income (loss)	640	(39)	1,941	805
Other comprehensive income (loss)
Unrealized appreciation	731	22	1,369	391
Reclassification adjustment for net realized gains included in net income	(56)	(18)	(147)	(152)
Other comprehensive income (loss)	675	4	1,222	239
Change in:
Cumulative translation adjustment	190	(152)	162	149
Pension liability	(4)	3	(5)	(2)
Other comprehensive income, before income tax	861	(145)	1,379	386
Income tax expense related to OCI items	(185)	(5)	(276)	(141)
Other comprehensive income (loss)	676	(150)	1,103	245
Comprehensive income (loss)	$ 1,316	$ (189)	$ 3,044	$ 1,050
Earnings per share
Basic earnings per share (US$ per share)	$ 1.88	$ (0.11)	$ 5.71	$ 2.38
Diluted earnings per share (US$ per share)	$ 1.86	$ (0.11)	$ 5.67	$ 2.36