Segment information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2026 USD ($)	Mar. 31, 2025 USD ($)
Segment Reporting Information [Line Items]
Net premiums written	$ 14,005	$ 12,646
Net premiums earned	13,457	12,000
Losses and loss expenses	6,131	6,896
Policy acquisition costs	2,596	2,313
Administrative expenses	1,149	1,080
Other (income) expense	(161)	(83)
Amortization of purchased intangibles	73	75
Segment Income (loss)	3,721	2,231
Net realized gains (losses)	(407)	(116)
Market risk benefits gains (losses)	14	(92)
Interest Income (Expense), Net	198	181
Integration expenses and severance	9	0
Segment Reporting, Other Segment Item, Amount	24	38
Income before income tax	$ 2,993	$ 1,664
Number of Reportable Segments	6	6
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	$ 4,895	$ 4,787
Net premiums earned	5,148	4,988
Losses and loss expenses	3,220	3,031
Segment Liability for Future Policy Benefits, Period Expense (Income)	0	0
Policy acquisition costs	752	719
Administrative expenses	354	344
Underwriting Income (Loss)	822	894
Segment Net Investment Income	971	929
Other (income) expense	14	8
Amortization of purchased intangibles	1	1
Segment Income (loss)	1,778	1,814
North American Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,681	1,552
Net premiums earned	1,746	1,574
Losses and loss expenses	1,034	2,093
Segment Liability for Future Policy Benefits, Period Expense (Income)	0	0
Policy acquisition costs	347	330
Administrative expenses	85	87
Underwriting Income (Loss)	280	(936)
Segment Net Investment Income	137	120
Other (income) expense	3	1
Amortization of purchased intangibles	2	2
Segment Income (loss)	412	(819)
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	311	276
Net premiums earned	189	165
Losses and loss expenses	53	92
Segment Liability for Future Policy Benefits, Period Expense (Income)	0	0
Policy acquisition costs	24	17
Administrative expenses	(6)	2
Underwriting Income (Loss)	118	54
Segment Net Investment Income	26	24
Other (income) expense	0	1
Amortization of purchased intangibles	6	6
Segment Income (loss)	138	71
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	4,466	3,903
Net premiums earned	3,780	3,209
Losses and loss expenses	1,652	1,397
Segment Liability for Future Policy Benefits, Period Expense (Income)	113	113
Policy acquisition costs	1,009	837
Administrative expenses	387	330
Underwriting Income (Loss)	619	532
Segment Net Investment Income	300	281
Other (income) expense	6	6
Amortization of purchased intangibles	22	19
Segment Income (loss)	891	788
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	363	408
Net premiums earned	326	368
Losses and loss expenses	137	242
Segment Liability for Future Policy Benefits, Period Expense (Income)	0	0
Policy acquisition costs	102	100
Administrative expenses	9	10
Underwriting Income (Loss)	78	16
Segment Net Investment Income	108	70
Other (income) expense	0	0
Amortization of purchased intangibles	0	0
Segment Income (loss)	186	86
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	2,289	1,720
Net premiums earned	2,268	1,696
Losses and loss expenses	28	26
Segment Liability for Future Policy Benefits, Period Expense (Income)	1,700	1,163
Policy acquisition costs	362	310
Administrative expenses	210	202
Segment Net Investment Income	305	271
Other (income) expense	(51)	(35)
Amortization of purchased intangibles	8	10
Segment Income (loss)	316	291
Segment Corporate and Other
Segment Reporting Information [Line Items]
Underwriting Income (Loss)	(125)	(119)
Segment Net Investment Income	(11)	(27)
Other (income) expense	(18)	33
Amortization of purchased intangibles	$ 34	$ 37