Consolidated Statements Of Operations and Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Revenues
Net premiums written	$ 6,710	$ 5,995
Decrease in unearned premiums	62	602
Net premiums earned	6,772	6,597
Net investment income	745	674
Net Realized Gains Losses [Abstract]
Other-than-temporary impairment (OTTI) losses gross	(19)	(71)
Portion of OTTI losses recognized in other comprehensive income (OCI)	0	8
Net OTTI losses recognized in income	(19)	(63)
Net realized gains (losses) excluding OTTI losses	12	(331)
Total net realized gains (losses) (includes $(8) and $(152) reclassified from AOCI)	(7)	(394)
Total revenues	7,510	6,877
Expenses
Losses and loss expenses	3,789	3,674
Policy benefits	168	126
Policy acquisition costs	1,397	1,413
Administrative expenses	676	772
Interest expense	154	146
Other (income) expense	(70)	28
Amortization of purchased intangibles	64	7
Chubb integration expenses	111	148
Total expenses	6,289	6,314
Income before income tax	1,221	563
Income tax (benefit) expense (includes $(6) and $(1) on reclassified unrealized losses)	128	124
Net income	1,093	439
Other comprehensive income
Unrealized appreciation	307	905
Reclassification adjustment for net realized losses included in net income	8	152
Unrealized appreciation (Depreciation) after reclassification adjustment	315	1,057
Change in:
Cumulative foreign currency translation adjustment	134	312
Postretirement benefit liability adjustment	(20)	2
Other comprehensive income, before income tax	429	1,371
Income tax expense related to OCI items	(115)	(269)
Other comprehensive income	314	1,102
Comprehensive income	$ 1,407	$ 1,541
Earnings per share
Basic earnings per share	$ 2.33	$ 0.98
Diluted earnings per share	$ 2.31	$ 0.97