Segment information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information [Line Items]
Net premiums written	$ 12,646	$ 12,221
Net premiums earned	12,000	11,583
Losses and loss expenses	6,896	5,727
Policy acquisition costs	2,313	2,207
Administrative expenses	1,080	1,070
Other (income) expense	(83)	(191)
Amortization of purchased intangibles	75	80
Segment Income (loss)	2,231	3,017
Net realized gains (losses)	(116)	(101)
Market risk benefits gains (losses)	(92)	21
Interest Income (Expense), Net	181	178
Integration expenses	0	7
Segment Reporting, Other Segment Item, Amount	38	1
Income before income tax	1,664	2,636
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	4,787	4,689
Net premiums earned	4,988	4,880
Losses and loss expenses	3,031	3,175
Segment Liability for Future Policy Benefits, Period Expense (Income)	0	0
Policy acquisition costs	719	688
Administrative expenses	344	328
Underwriting Income (Loss)	894	689
Segment Net Investment Income	929	826
Other (income) expense	8	7
Amortization of purchased intangibles	1	0
Segment Income (loss)	1,814	1,508
North American Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,552	1,456
Net premiums earned	1,574	1,471
Losses and loss expenses	2,093	899
Segment Liability for Future Policy Benefits, Period Expense (Income)	0	0
Policy acquisition costs	330	300
Administrative expenses	87	86
Underwriting Income (Loss)	(936)	186
Segment Net Investment Income	120	102
Other (income) expense	1	1
Amortization of purchased intangibles	2	2
Segment Income (loss)	(819)	285
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	276	249
Net premiums earned	165	128
Losses and loss expenses	92	49
Segment Liability for Future Policy Benefits, Period Expense (Income)	0	0
Policy acquisition costs	17	21
Administrative expenses	2	2
Underwriting Income (Loss)	54	56
Segment Net Investment Income	24	21
Other (income) expense	1	0
Amortization of purchased intangibles	6	6
Segment Income (loss)	71	71
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	3,903	3,835
Net premiums earned	3,209	3,198
Losses and loss expenses	1,397	1,426
Segment Liability for Future Policy Benefits, Period Expense (Income)	113	100
Policy acquisition costs	837	823
Administrative expenses	330	331
Underwriting Income (Loss)	532	518
Segment Net Investment Income	281	267
Other (income) expense	6	5
Amortization of purchased intangibles	19	20
Segment Income (loss)	788	760
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	408	359
Net premiums earned	368	295
Losses and loss expenses	242	137
Segment Liability for Future Policy Benefits, Period Expense (Income)	0	0
Policy acquisition costs	100	81
Administrative expenses	10	9
Underwriting Income (Loss)	16	68
Segment Net Investment Income	70	57
Other (income) expense	0	0
Amortization of purchased intangibles	0	0
Segment Income (loss)	86	125
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,720	1,633
Net premiums earned	1,696	1,611
Losses and loss expenses	26	32
Segment Liability for Future Policy Benefits, Period Expense (Income)	1,163	1,070
Policy acquisition costs	310	294
Administrative expenses	202	207
Segment Net Investment Income	271	230
Other (income) expense	(35)	(40)
Amortization of purchased intangibles	10	10
Segment Income (loss)	291	268
Segment Corporate and Other
Segment Reporting Information [Line Items]
Underwriting Income (Loss)	(119)	(117)
Segment Net Investment Income	(27)	(26)
Other (income) expense	33	(68)
Amortization of purchased intangibles	$ 37	$ 42