Consolidated Statements Of Operations and Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenues
Net premiums written	$ 7,902	$ 7,573	$ 22,193	$ 21,207
Decrease (increase) in unearned premiums	(95)	115	(377)	483
Net premiums earned	7,807	7,688	21,816	21,690
Net investment income	813	739	2,328	2,121
Net Realized Gains Losses [Abstract]
Other-than-temporary impairment (OTTI) losses gross	(8)	(12)	(36)	(99)
Portion of OTTI losses recognized in other comprehensive income (OCI)	0	0	1	8
Net OTTI losses recognized in income	(8)	(12)	(35)	(91)
Net realized gains (losses) excluding OTTI losses	(2)	112	119	(419)
Total net realized gains (losses) (includes $10, $33, $27, and $(117) reclassified from AOCI)	(10)	100	84	(510)
Total revenues	8,610	8,527	24,228	23,301
Expenses
Losses and loss expenses	6,247	4,269	14,182	12,197
Policy benefits	169	155	500	427
Policy acquisition costs	1,488	1,514	4,334	4,487
Administrative expenses	714	772	2,096	2,373
Interest expense	150	152	451	451
Other (income) expense	(118)	(91)	(333)	(92)
Amortization of purchased intangibles	65	4	194	16
Chubb integration expenses	50	115	233	361
Total expenses	8,765	6,890	21,657	20,220
Loss before income tax	(155)	1,637	2,571	3,081
Income tax expense (benefit) (includes nil, $11, $3 and $16 on reclassified unrealized gains and losses)	(85)	277	243	556
Net income (loss)	(70)	1,360	2,328	2,525
Other comprehensive income
Unrealized appreciation	153	247	919	2,099
Reclassification adjustment for net realized (gains) losses included in net income (loss)	(10)	(33)	(27)	117
Unrealized appreciation (Depreciation) after reclassification adjustment	143	214	892	2,216
Change in:
Cumulative foreign currency translation adjustment	665	(124)	901	269
Postretirement benefit liability adjustment	(63)	5	(118)	8
Other comprehensive income, before income tax	745	95	1,675	2,493
Income tax expense related to OCI items	(46)	(79)	(292)	(561)
Other comprehensive income	699	16	1,383	1,932
Comprehensive income	$ 629	$ 1,376	$ 3,711	$ 4,457
Earnings (loss) per share
Basic earnings (loss) per share	$ (0.15)	$ 2.90	$ 4.98	$ 5.48
Diluted earnings (loss) per share	$ (0.15)	$ 2.88	$ 4.94	$ 5.44