Segment information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting Information [Line Items]
Net premiums written	$ 14,866	$ 13,829	$ 41,708	$ 39,410
Net premiums earned	14,359	13,373	39,484	37,248
Losses and loss expenses	6,951	7,383	20,419	19,541
Policy acquisition costs	2,563	2,324	7,291	6,757
Administrative expenses	1,138	1,094	3,343	3,258
Other (income) expense	(43)	(325)	(781)	(626)
Amortization of purchased intangibles	75	81	224	241
Segment Income (loss)	4,218	3,272	9,957	9,462
Net realized gains (losses)	283	198	327	201
Market risk benefits gains (losses)	(142)	(230)	(251)	(238)
Interest Income (Expense), Net	197	192	559	552
Integration expenses	1	7	3	21
Segment Reporting, Other Segment Item, Amount	48	(19)	48	(100)
Income before income tax	3,894	2,994	9,274	8,336
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	5,663	5,500	16,173	15,690
Net premiums earned	5,080	5,110	15,245	14,890
Losses and loss expenses	3,083	3,391	9,372	9,640
Segment Liability for Future Policy Benefits, Period Expense (Income)	0	0	0	0
Policy acquisition costs	708	689	2,132	2,037
Administrative expenses	348	338	1,049	993
Underwriting Income (Loss)	941	692	2,692	2,220
Segment Net Investment Income	978	931	2,845	2,620
Other (income) expense	10	6	26	28
Amortization of purchased intangibles	1	2	4	2
Segment Income (loss)	1,908	1,615	5,507	4,810
North American Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,814	1,679	5,304	4,911
Net premiums earned	1,741	1,577	4,996	4,560
Losses and loss expenses	713	879	3,628	2,654
Segment Liability for Future Policy Benefits, Period Expense (Income)	0	0	0	0
Policy acquisition costs	339	315	1,001	914
Administrative expenses	82	88	251	262
Underwriting Income (Loss)	607	295	116	730
Segment Net Investment Income	123	112	361	322
Other (income) expense	1	1	2	0
Amortization of purchased intangibles	2	3	6	7
Segment Income (loss)	727	403	469	1,045
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,458	1,379	2,467	2,386
Net premiums earned	1,495	1,419	2,258	2,173
Losses and loss expenses	1,224	1,193	1,799	1,785
Segment Liability for Future Policy Benefits, Period Expense (Income)	0	0	0	0
Policy acquisition costs	86	88	151	154
Administrative expenses	5	2	9	7
Underwriting Income (Loss)	180	136	299	227
Segment Net Investment Income	20	20	63	62
Other (income) expense	0	1	1	1
Amortization of purchased intangibles	6	5	18	18
Segment Income (loss)	194	150	343	270
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	3,695	3,367	11,218	10,536
Net premiums earned	3,803	3,421	10,554	9,966
Losses and loss expenses	1,707	1,631	4,893	4,728
Segment Liability for Future Policy Benefits, Period Expense (Income)	119	120	361	312
Policy acquisition costs	975	852	2,725	2,517
Administrative expenses	368	340	1,067	1,019
Underwriting Income (Loss)	634	478	1,508	1,390
Segment Net Investment Income	288	286	847	836
Other (income) expense	8	5	19	14
Amortization of purchased intangibles	20	21	58	61
Segment Income (loss)	894	738	2,278	2,151
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	304	352	1,092	1,122
Net premiums earned	315	316	1,021	950
Losses and loss expenses	139	200	513	492
Segment Liability for Future Policy Benefits, Period Expense (Income)	0	0	0	0
Policy acquisition costs	96	89	294	250
Administrative expenses	9	9	29	29
Underwriting Income (Loss)	71	18	185	179
Segment Net Investment Income	100	64	255	179
Other (income) expense	0	0	0	0
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	171	82	440	358
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,932	1,552	5,454	4,765
Net premiums earned	1,925	1,530	5,410	4,709
Losses and loss expenses	31	32	77	86
Segment Liability for Future Policy Benefits, Period Expense (Income)	1,303	989	3,715	3,090
Policy acquisition costs	359	291	988	885
Administrative expenses	213	213	614	638
Segment Net Investment Income	284	250	829	738
Other (income) expense	(31)	(39)	(103)	(111)
Amortization of purchased intangibles	10	10	28	31
Segment Income (loss)	324	284	920	828
Segment Corporate and Other
Segment Reporting Information [Line Items]
Underwriting Income (Loss)	(174)	(162)	(469)	(471)
Segment Net Investment Income	(18)	(28)	(74)	(86)
Other (income) expense	87	(202)	(408)	(263)
Amortization of purchased intangibles	$ 36	$ 40	$ 110	$ 122