Segment information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Net premiums written	$ 13,829	$ 13,104	$ 39,410	$ 35,765
Net premiums earned	13,373	12,674	37,248	33,815
Losses and loss expenses	7,383	7,106	19,541	17,937
Policy benefits	1,099	938	3,498	2,565
Policy acquisition costs	2,324	2,178	6,757	6,142
Administrative expenses	1,094	1,060	3,258	2,959
Underwriting income (loss)	1,473	1,392	4,194	4,212
Net investment income (loss)	1,508	1,314	4,367	3,566
Other (income) expense	(325)	(154)	(626)	(550)
Amortization of purchased intangibles	81	84	241	226
Segment Income (loss)	3,225	2,776	8,946	8,102
Net realized gains (losses)	198	(103)	201	(484)
Market risk benefits gains (losses)	(230)	(32)	(238)	(154)
Interest Income (Expense), Net	192	174	552	499
Integration expenses	7	14	21	51
Income tax expense	504	413	1,336	1,189
Net income	2,490	2,040	7,000	5,725
Net income (loss) attributable to noncontrolling interests	166	(3)	303	(3)
Net income (loss)	2,324	2,043	6,697	5,728
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	5,500	5,132	15,690	14,575
Net premiums earned	5,110	4,735	14,890	13,710
Losses and loss expenses	3,391	3,025	9,640	8,625
Policy benefits	0	0	0	0
Policy acquisition costs	689	640	2,037	1,867
Administrative expenses	338	323	993	934
Underwriting income (loss)	692	747	2,220	2,284
Net investment income (loss)	931	780	2,620	2,204
Other (income) expense	6	6	28	18
Amortization of purchased intangibles	2	0	2	0
Segment Income (loss)	1,615	1,521	4,810	4,470
North American Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,679	1,527	4,911	4,404
Net premiums earned	1,577	1,407	4,560	4,084
Losses and loss expenses	879	900	2,654	2,634
Policy benefits	0	0	0	0
Policy acquisition costs	315	287	914	836
Administrative expenses	88	84	262	247
Underwriting income (loss)	295	136	730	367
Net investment income (loss)	112	94	322	262
Other (income) expense	1	2	0	2
Amortization of purchased intangibles	3	3	7	8
Segment Income (loss)	403	225	1,045	619
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,379	1,521	2,386	2,581
Net premiums earned	1,419	1,540	2,173	2,334
Losses and loss expenses	1,193	1,356	1,785	2,003
Policy benefits	0	0	0	0
Policy acquisition costs	88	76	154	128
Administrative expenses	2	3	7	9
Underwriting income (loss)	136	105	227	194
Net investment income (loss)	20	12	62	43
Other (income) expense	1	0	1	0
Amortization of purchased intangibles	5	6	18	19
Segment Income (loss)	150	111	270	218
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	3,367	3,211	10,536	9,359
Net premiums earned	3,421	3,311	9,966	9,005
Losses and loss expenses	1,631	1,635	4,728	4,139
Policy benefits	120	91	312	338
Policy acquisition costs	852	827	2,517	2,286
Administrative expenses	340	327	1,019	899
Underwriting income (loss)	478	431	1,390	1,343
Net investment income (loss)	286	248	836	636
Other (income) expense	5	(10)	14	(29)
Amortization of purchased intangibles	21	19	61	52
Segment Income (loss)	738	670	2,151	1,956
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	352	261	1,122	831
Net premiums earned	316	239	950	720
Losses and loss expenses	200	116	492	319
Policy benefits	0	0	0	0
Policy acquisition costs	89	69	250	196
Administrative expenses	9	9	29	27
Underwriting income (loss)	18	45	179	178
Net investment income (loss)	64	47	179	144
Other (income) expense	0	0	0	(1)
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	82	92	358	323
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,552	1,452	4,765	4,015
Net premiums earned	1,530	1,442	4,709	3,962
Losses and loss expenses	32	20	86	87
Policy benefits	989	866	3,090	2,283
Policy acquisition costs	291	279	885	829
Administrative expenses	213	216	638	553
Underwriting income (loss)	5	61	10	210
Net investment income (loss)	250	211	738	525
Other (income) expense	(39)	(28)	(111)	(69)
Amortization of purchased intangibles	10	12	31	18
Segment Income (loss)	284	288	828	786
Segment Corporate and Other
Segment Reporting Information [Line Items]
Net premiums written	0	0	0	0
Net premiums earned	0	0	0	0
Losses and loss expenses	58	61	161	133
Policy benefits	0	0	0	0
Policy acquisition costs	0	0	0	0
Administrative expenses	104	98	310	290
Underwriting income (loss)	(162)	(159)	(471)	(423)
Net investment income (loss)	(28)	14	(86)	28
Other (income) expense	(202)	(51)	(263)	(251)
Amortization of purchased intangibles	40	44	122	129
Segment Income (loss)	(28)	(138)	(416)	(273)
Net realized gains (losses)	179	(96)	101	(481)
Market risk benefits gains (losses)	(230)	(32)	(238)	(154)
Interest Income (Expense), Net	192	174	552	499
Integration expenses	7	14	21	51
Income tax expense	504	413	1,336	1,189
Net income	(782)	(867)	(2,462)	(2,647)
Net income (loss) attributable to noncontrolling interests	166	(3)	303	(3)
Net income (loss)	(948)	(864)	(2,765)	(2,644)
Segment Measure Reconciling Items
Segment Reporting Information [Line Items]
Net premiums written	0	0	0	0
Net premiums earned	0	0	0	0
Losses and loss expenses	(1)	(7)	(5)	(3)
Policy benefits	(10)	(19)	96	(56)
Policy acquisition costs	0	0	0	0
Administrative expenses	0	0	0	0
Underwriting income (loss)	11	26	(91)	59
Net investment income (loss)	(127)	(92)	(304)	(276)
Other (income) expense	(97)	(73)	(295)	(220)
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	(19)	7	(100)	3
Net realized gains (losses)	19	(7)	100	(3)
Market risk benefits gains (losses)	0	0	0	0
Interest Income (Expense), Net	0	0	0	0
Integration expenses	0	0	0	0
Income tax expense	0	0	0	0
Net income	0	0	0	0
Net income (loss) attributable to noncontrolling interests	0	0	0	0
Net income (loss)	$ 0	$ 0	$ 0	$ 0