Consolidated Statements Of Operations and Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenues
Net premiums written	$ 7,581	$ 7,639	$ 14,291	$ 13,634
Decrease (increase) in unearned premiums	(344)	(234)	(282)	368
Net premiums earned	7,237	7,405	14,009	14,002
Net investment income	770	708	1,515	1,382
Net Realized Gains Losses [Abstract]
Other-than-temporary impairment (OTTI) losses gross	(9)	(16)	(28)	(87)
Portion of OTTI losses recognized in other comprehensive income (OCI)	1	0	1	8
Net OTTI losses recognized in income	(8)	(16)	(27)	(79)
Net realized gains (losses) excluding OTTI losses	109	(200)	121	(531)
Total net realized gains (losses) (includes $25, $2, $17, and $(150) reclassified from AOCI)	101	(216)	94	(610)
Total revenues	8,108	7,897	15,618	14,774
Expenses
Losses and loss expenses	4,146	4,254	7,935	7,928
Policy benefits	163	146	331	272
Policy acquisition costs	1,449	1,560	2,846	2,973
Administrative expenses	706	829	1,382	1,601
Interest expense	147	153	301	299
Other (income) expense	(145)	(29)	(215)	(1)
Amortization of purchased intangibles	65	5	129	12
Chubb integration expenses	72	98	183	246
Total expenses	6,603	7,016	12,892	13,330
Income before income tax	1,505	881	2,726	1,444
Income tax expense (includes $9, $6, $3 and $5 on reclassified unrealized gains and losses)	200	155	328	279
Net income	1,305	726	2,398	1,165
Other comprehensive income
Unrealized appreciation	459	947	766	1,852
Reclassification adjustment for net realized (gains) losses included in net income	(25)	(2)	(17)	150
Unrealized appreciation (Depreciation) after reclassification adjustment	434	945	749	2,002
Change in:
Cumulative foreign currency translation adjustment	102	81	236	393
Postretirement benefit liability adjustment	(35)	1	(55)	3
Other comprehensive income, before income tax	501	1,027	930	2,398
Income tax expense related to OCI items	(131)	(213)	(246)	(482)
Other comprehensive income	370	814	684	1,916
Comprehensive income	$ 1,675	$ 1,540	$ 3,082	$ 3,081
Earnings per share
Basic earnings per share	$ 2.79	$ 1.55	$ 5.12	$ 2.55
Diluted earnings per share	$ 2.77	$ 1.54	$ 5.08	$ 2.53