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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Revenues
Net premiums written	$ 4,729	$ 4,620	$ 13,473	$ 12,809
Decrease (increase) in unearned premiums	25	(10)	(417)	(559)
Net premiums earned	4,754	4,610	13,056	12,250
Net investment income	566	522	1,675	1,587
Net realized gains (losses):
Other-than-temporary impairment (OTTI) losses gross	(5)	(4)	(30)	(14)
Portion of OTTI losses recognized in other comprehensive income (OCI)	1	0	3	0
Net OTTI losses recognized in income	(4)	(4)	(27)	(14)
Net realized gains (losses) excluding OTTI losses	(116)	44	(270)	364
Total net realized gains (losses)	(120)	40	(297)	350
Total revenues	5,200	5,172	14,434	14,187
Expenses
Losses and loss expenses	2,684	2,655	7,233	6,831
Policy benefits	125	138	383	379
Policy acquisition costs	825	678	2,311	1,957
Administrative expenses	554	563	1,655	1,641
Interest expense	70	72	213	205
Other (income) expense	(19)	(5)	(61)	22
Total expenses	4,239	4,101	11,734	11,035
Income before income tax	961	1,071	2,700	3,152
Income tax expense (includes $5, $4, $7, and $16 on reclassified unrealized gains and losses)	176	155	402	392
Net income	785	916	2,298	2,760
Other comprehensive income (loss)
Unrealized appreciation (depreciation)	(375)	16	733	(1,570)
Reclassification adjustment for net realized (gains) losses included in net income	38	(24)	11	(97)
Unrealized appreciation (Depreciation) after reclassification adjustment	(337)	(8)	744	(1,667)
Change in:
Cumulative translation adjustment	(251)	149	(131)	(237)
Pension liability	7	(2)	(6)	17
Other comprehensive income (loss), before income tax	(581)	139	607	(1,887)
Income tax (expense) benefit related to OCI items	94	(25)	(151)	374
Other comprehensive income (loss)	(487)	114	456	(1,513)
Comprehensive income	298	1,030	2,754	1,247
Earnings per share
Basic earnings per share	$ 2.35	$ 2.68	$ 6.82	$ 8.09
Diluted earnings per share	$ 2.32	$ 2.66	$ 6.75	$ 8.02
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Net realized gains (losses):
Total net realized gains (losses)	(38)	24	(11)	97
Expenses
Income tax expense (includes $5, $4, $7, and $16 on reclassified unrealized gains and losses)	$ 5	$ 4	$ 7	$ 16