Consolidated Statements Of Operations and Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenues
Net premiums written	$ 8,355	$ 8,343	$ 16,332	$ 15,656
Increase in unearned premiums	(227)	(452)	(410)	(628)
Net premiums earned	8,128	7,891	15,922	15,028
Net investment income	827	859	1,688	1,695
Net realized gains (losses):
Other-than-temporary impairment (OTTI) losses gross	0	(14)	0	(27)
Portion of OTTI losses recognized in other comprehensive income (OCI)	0	1	0	1
Net OTTI losses recognized in income	0	(13)	0	(26)
Net realized gains (losses) excluding OTTI losses	0	(210)	0	(294)
Net Realized Gains Losses	30	(223)	(928)	(320)
Total revenues	8,985	8,527	16,682	16,403
Expenses
Losses and loss expenses	6,577	4,715	11,062	8,813
Policy benefits	223	161	352	357
Policy acquisition costs	1,593	1,544	3,208	3,008
Administrative expenses	727	758	1,468	1,468
Interest expense	128	140	260	280
Other (income) expense	58	(230)	113	(269)
Amortization of purchased intangibles	72	77	145	153
Chubb integration expenses	0	4	0	7
Total expenses	9,378	7,169	16,608	13,817
Income (loss) before income tax	(393)	1,358	74	2,586
Income tax expense (benefit) (includes $(2), $4, $(42), and $(2) on reclassified unrealized gains and losses)	(62)	208	153	396
Net income (loss)	(331)	1,150	(79)	2,190
Other comprehensive income
Unrealized appreciation (depreciation)	3,248	1,252	769	3,097
Reclassification adjustment for net realized (gains) losses included in net income (loss)	33	(12)	352	32
Unrealized appreciation (Depreciation) after reclassification adjustment	3,281	1,240	1,121	3,129
Change in:
Cumulative foreign currency translation adjustment	445	(97)	(414)	50
Postretirement benefit liability adjustment	(22)	(18)	(36)	(45)
Other comprehensive income, before income tax	3,704	1,125	671	3,134
Income tax (expense) benefit related to OCI items	(513)	(216)	(192)	(547)
Other comprehensive income	3,191	909	479	2,587
Comprehensive income	$ 2,860	$ 2,059	$ 400	$ 4,777
Earnings per share
Basic earnings (loss) per share	$ (0.73)	$ 2.52	$ (0.17)	$ 4.78
Diluted earnings (loss) per share	$ (0.73)	$ 2.50	$ (0.17)	$ 4.75
Total net realized gains (losses) reclassified from AOCI	$ (33)	$ 12	$ (352)	$ (32)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	(2)	4	(42)	(2)
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
Earnings per share
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	$ (2)	$ 4	$ (42)	$ (2)