Consolidated Statements Of Operations and Comprehensive Income (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Revenues
Net premiums written	$ 4,559	$ 4,391	$ 8,744	$ 8,189
Increase in unearned premiums	(227)	(324)	(442)	(549)
Net premiums earned	4,332	4,067	8,302	7,640
Net investment income	556	534	1,109	1,065
Net realized gains (losses):
Other-than-temporary impairment (OTTI) losses gross	(13)	(7)	(25)	(10)
Portion of OTTI losses recognized in other comprehensive income (OCI)	1	0	2	0
Net OTTI losses recognized in income	(12)	(7)	(23)	(10)
Net realized gains (losses) excluding OTTI losses	(61)	111	(154)	320
Total net realized gains (losses)	(73)	104	(177)	310
Total revenues	4,815	4,705	9,234	9,015
Expenses
Losses and loss expenses	2,388	2,250	4,549	4,176
Policy benefits	144	110	258	241
Policy acquisition costs	758	665	1,486	1,279
Administrative expenses	566	564	1,101	1,078
Interest expense	72	73	143	133
Other (income) expense	(25)	37	(42)	27
Total expenses	3,903	3,699	7,495	6,934
Income before income tax	912	1,006	1,739	2,081
Income tax expense (includes $5, $7, $2, and $12 of reclassified unrealized gains)	133	115	226	237
Net income	779	891	1,513	1,844
Other comprehensive income (loss)
Unrealized appreciation (depreciation)	589	(1,470)	1,108	(1,586)
Reclassification adjustment for net realized gains included in net income	(21)	(38)	(27)	(73)
Unrealized appreciation (Depreciation) after reclassification adjustment	568	(1,508)	1,081	(1,659)
Change in:
Cumulative translation adjustment	153	(175)	120	(386)
Pension liability	(5)	(3)	(13)	19
Other comprehensive income (loss), before income tax	716	(1,686)	1,188	(2,026)
Income tax (expense) benefit related to OCI items	(147)	296	(245)	399
Other comprehensive income (loss)	569	(1,390)	943	(1,627)
Comprehensive income (loss)	1,348	(499)	2,456	217
Earnings per share
Basic earnings per share	$ 2.30	$ 2.61	$ 4.47	$ 5.41
Diluted earnings per share	$ 2.28	$ 2.59	$ 4.43	$ 5.36
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Net realized gains (losses):
Total net realized gains (losses)	21	38	27	73
Expenses
Income tax expense (includes $5, $7, $2, and $12 of reclassified unrealized gains)	$ 5	$ 7	$ 2	$ 12