Segment information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Segment Reporting Information [Line Items]
Net premiums written	$ 14,196	$ 13,360	$ 26,842	$ 25,581
Net premiums earned	13,125	12,292	25,125	23,875
Losses and loss expenses	6,572	6,431	13,468	12,158
Policy acquisition costs	2,415	2,226	4,728	4,433
Administrative expenses	1,125	1,094	2,205	2,164
Other (income) expense	(655)	(110)	(738)	(301)
Amortization of purchased intangibles	74	80	149	160
Segment Income (loss)	3,508	3,173	5,739	6,190
Net realized gains (losses)	160	104	44	3
Market risk benefits gains (losses)	(17)	(29)	(109)	(8)
Interest Income (Expense), Net	181	182	362	360
Integration expenses	2	7	2	14
Segment Reporting, Other Segment Item, Amount	(38)	(82)		(81)
Income before income tax	3,716	2,706	5,380	5,342
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	5,723	5,501	10,510	10,190
Net premiums earned	5,177	4,900	10,165	9,780
Losses and loss expenses	3,258	3,074	6,289	6,249
Segment Liability for Future Policy Benefits, Period Expense (Income)	0	0	0	0
Policy acquisition costs	705	660	1,424	1,348
Administrative expenses	357	327	701	655
Underwriting Income (Loss)	857	839	1,751	1,528
Segment Net Investment Income	938	863	1,867	1,689
Other (income) expense	8	15	16	22
Amortization of purchased intangibles	2	0	3	0
Segment Income (loss)	1,785	1,687	3,599	3,195
North American Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,938	1,776	3,490	3,232
Net premiums earned	1,681	1,512	3,255	2,983
Losses and loss expenses	822	876	2,915	1,775
Segment Liability for Future Policy Benefits, Period Expense (Income)	0	0	0	0
Policy acquisition costs	332	299	662	599
Administrative expenses	82	88	169	174
Underwriting Income (Loss)	445	249	(491)	435
Segment Net Investment Income	118	108	238	210
Other (income) expense	0	(2)	1	(1)
Amortization of purchased intangibles	2	2	4	4
Segment Income (loss)	561	357	(258)	642
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	733	758	1,009	1,007
Net premiums earned	598	626	763	754
Losses and loss expenses	483	543	575	592
Segment Liability for Future Policy Benefits, Period Expense (Income)	0	0	0	0
Policy acquisition costs	48	45	65	66
Administrative expenses	2	3	4	5
Underwriting Income (Loss)	65	35	119	91
Segment Net Investment Income	19	21	43	42
Other (income) expense	0	0	1	0
Amortization of purchased intangibles	6	7	12	13
Segment Income (loss)	78	49	149	120
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	3,620	3,334	7,523	7,169
Net premiums earned	3,542	3,347	6,751	6,545
Losses and loss expenses	1,789	1,671	3,186	3,097
Segment Liability for Future Policy Benefits, Period Expense (Income)	129	92	242	192
Policy acquisition costs	913	842	1,750	1,665
Administrative expenses	369	348	699	679
Underwriting Income (Loss)	342	394	874	912
Segment Net Investment Income	278	283	559	550
Other (income) expense	5	4	11	9
Amortization of purchased intangibles	19	20	38	40
Segment Income (loss)	596	653	1,384	1,413
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	380	411	788	770
Net premiums earned	338	339	706	634
Losses and loss expenses	132	155	374	292
Segment Liability for Future Policy Benefits, Period Expense (Income)	0	0	0	0
Policy acquisition costs	98	80	198	161
Administrative expenses	10	11	20	20
Underwriting Income (Loss)	98	93	114	161
Segment Net Investment Income	85	58	155	115
Other (income) expense	0	0	0	0
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	183	151	269	276
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,802	1,580	3,522	3,213
Net premiums earned	1,789	1,568	3,485	3,179
Losses and loss expenses	20	22	46	54
Segment Liability for Future Policy Benefits, Period Expense (Income)	1,249	1,031	2,412	2,101
Policy acquisition costs	319	300	629	594
Administrative expenses	199	218	401	425
Segment Net Investment Income	274	258	545	488
Other (income) expense	(37)	(32)	(72)	(72)
Amortization of purchased intangibles	8	11	18	21
Segment Income (loss)	305	276	596	544
Segment Corporate and Other
Segment Reporting Information [Line Items]
Underwriting Income (Loss)	(176)	(192)	(295)	(309)
Segment Net Investment Income	(29)	(32)	(56)	(58)
Other (income) expense	(528)	7	(495)	(61)
Amortization of purchased intangibles	$ 37	$ 40	$ 74	$ 82