Segment information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Net premiums written	$ 12,221	$ 10,710
Net premiums earned	11,583	10,142
Losses and loss expenses	5,727	5,148
Policy benefits (includes remeasurement gains (losses) of $(19) and $1)	1,180	797
Policy acquisition costs	2,207	1,948
Administrative expenses	1,070	930
Underwriting income (loss)	1,399	1,319
Net investment income (loss)	1,391	1,107
Other (income) expense	(191)	(296)
Amortization of purchased intangibles	80	72
Segment Income (loss)	2,901	2,650
Net realized gains (losses)	(101)	(77)
Market risk benefits gains (losses)	21	(115)
Interest Income (Expense), Net	178	160
Cigna integration expenses	7	22
Income tax expense	342	384
Net Income	2,294	1,892
Net income attributable to noncontrolling interests	151	0
Net income (loss)	2,143	1,892
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	4,689	4,288
Net premiums earned	4,880	4,369
Losses and loss expenses	3,175	2,729
Policy benefits (includes remeasurement gains (losses) of $(19) and $1)	0	0
Policy acquisition costs	688	613
Administrative expenses	328	295
Underwriting income (loss)	689	732
Net investment income (loss)	826	698
Other (income) expense	7	7
Amortization of purchased intangibles	0	0
Segment Income (loss)	1,508	1,423
North American Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,456	1,296
Net premiums earned	1,471	1,320
Losses and loss expenses	899	888
Policy benefits (includes remeasurement gains (losses) of $(19) and $1)	0	0
Policy acquisition costs	300	272
Administrative expenses	86	79
Underwriting income (loss)	186	81
Net investment income (loss)	102	82
Other (income) expense	1	1
Amortization of purchased intangibles	2	2
Segment Income (loss)	285	160
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	249	293
Net premiums earned	128	159
Losses and loss expenses	49	140
Policy benefits (includes remeasurement gains (losses) of $(19) and $1)	0	0
Policy acquisition costs	21	15
Administrative expenses	2	3
Underwriting income (loss)	56	1
Net investment income (loss)	21	17
Other (income) expense	0	1
Amortization of purchased intangibles	6	6
Segment Income (loss)	71	11
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	3,835	3,263
Net premiums earned	3,198	2,786
Losses and loss expenses	1,426	1,237
Policy benefits (includes remeasurement gains (losses) of $(19) and $1)	100	110
Policy acquisition costs	823	713
Administrative expenses	331	280
Underwriting income (loss)	518	446
Net investment income (loss)	267	188
Other (income) expense	5	(9)
Amortization of purchased intangibles	20	18
Segment Income (loss)	760	625
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	359	277
Net premiums earned	295	244
Losses and loss expenses	137	112
Policy benefits (includes remeasurement gains (losses) of $(19) and $1)	0	0
Policy acquisition costs	81	62
Administrative expenses	9	9
Underwriting income (loss)	68	61
Net investment income (loss)	57	49
Other (income) expense	0	(1)
Amortization of purchased intangibles	0	0
Segment Income (loss)	125	111
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,633	1,293
Net premiums earned	1,611	1,264
Losses and loss expenses	32	32
Policy benefits (includes remeasurement gains (losses) of $(19) and $1)	1,070	712
Policy acquisition costs	294	273
Administrative expenses	207	167
Underwriting income (loss)	8	80
Net investment income (loss)	230	153
Other (income) expense	(40)	(15)
Amortization of purchased intangibles	10	4
Segment Income (loss)	268	244
Segment Corporate and Other
Segment Reporting Information [Line Items]
Net premiums written	0	0
Net premiums earned	0	0
Losses and loss expenses	10	11
Policy benefits (includes remeasurement gains (losses) of $(19) and $1)	0	0
Policy acquisition costs	0	0
Administrative expenses	107	97
Underwriting income (loss)	(117)	(108)
Net investment income (loss)	(26)	11
Other (income) expense	(68)	(214)
Amortization of purchased intangibles	42	42
Segment Income (loss)	(117)	75
Net realized gains (losses)	(100)	(76)
Market risk benefits gains (losses)	21	(115)
Interest Income (Expense), Net	178	160
Cigna integration expenses	7	22
Income tax expense	342	384
Net Income	(723)	(682)
Net income attributable to noncontrolling interests	151
Net income (loss)	(874)
Segment Measure Reconciling Items
Segment Reporting Information [Line Items]
Net premiums written	0	0
Net premiums earned	0	0
Losses and loss expenses	(1)	(1)
Policy benefits (includes remeasurement gains (losses) of $(19) and $1)	10	(25)
Policy acquisition costs	0	0
Administrative expenses		0
Underwriting income (loss)	(9)	26
Net investment income (loss)	(86)	(91)
Other (income) expense	(96)	(66)
Amortization of purchased intangibles	0	0
Segment Income (loss)	1	1
Net realized gains (losses)	(1)	(1)
Market risk benefits gains (losses)	0	0
Interest Income (Expense), Net	0	0
Cigna integration expenses	0	0
Income tax expense	0	0
Net Income	0	$ 0
Net income attributable to noncontrolling interests	0
Net income (loss)	$ 0