Consolidated Statements Of Operations and Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Revenues
Net premiums written	$ 4,709	$ 4,729	$ 13,569	$ 13,473
Decrease (increase) in unearned premiums	10	25	(563)	(417)
Net premiums earned	4,719	4,754	13,006	13,056
Net investment income	549	566	1,662	1,675
Net realized gains (losses):
Other-than-temporary impairment (OTTI) losses gross	(35)	(5)	(62)	(30)
Portion of OTTI losses recognized in other comprehensive income (OCI)	5	1	11	3
Net OTTI losses recognized in income	(30)	(4)	(51)	(27)
Net realized gains (losses) excluding OTTI losses	(367)	(116)	(309)	(270)
Total net realized gains (losses) (includes $(49), $(38), $(18), and $(11) reclassified from AOCI)	(397)	(120)	(360)	(297)
Total revenues	4,871	5,200	14,308	14,434
Expenses
Losses and loss expenses	2,643	2,684	7,182	7,233
Policy benefits	89	125	384	383
Policy acquisition costs	771	825	2,205	2,311
Administrative expenses	568	554	1,700	1,655
Interest expense	68	70	207	213
Other (income) expense	12	(46)	(61)	(139)
Amortization of intangible assets	51	27	136	78
Chubb integration expenses	9	0	9	0
Total expenses	4,211	4,239	11,762	11,734
Income before income tax	660	961	2,546	2,700
Income tax expense (includes $(4), $5, $10, and $7 on reclassified unrealized gains and losses)	132	176	395	402
Net income	528	785	2,151	2,298
Other comprehensive income (loss)
Unrealized appreciation (depreciation)	(321)	(375)	(696)	733
Reclassification adjustment for net realized losses included in net income	49	38	18	11
Unrealized appreciation (Depreciation) after reclassification adjustment	(272)	(337)	(678)	744
Change in:
Cumulative translation adjustment	(575)	(251)	(860)	(131)
Pension liability	3	7	10	(6)
Other comprehensive income (loss), before income tax	(844)	(581)	(1,528)	607
Income tax (expense) benefit related to OCI items	45	94	145	(151)
Other comprehensive income (loss)	(799)	(487)	(1,383)	456
Comprehensive income (loss)	$ (271)	$ 298	$ 768	$ 2,754
Earnings per share
Basic earnings per share	$ 1.63	$ 2.35	$ 6.60	$ 6.82
Diluted earnings per share	$ 1.62	$ 2.32	$ 6.53	$ 6.75
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Net realized gains (losses):
Total net realized gains (losses) (includes $(49), $(38), $(18), and $(11) reclassified from AOCI)	$ (49)	$ (38)	$ (18)	$ (11)
Expenses
Income tax expense (includes $(4), $5, $10, and $7 on reclassified unrealized gains and losses)	$ (4)	$ 5	$ 10	$ 7