Segment information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Net premiums written	$ 13,104	$ 12,012	$ 35,765	$ 31,494
Net premiums earned	12,674	11,530	33,815	29,816
Losses and loss expenses	7,106	7,063	17,937	16,833
Policy benefits (includes remeasurement gains of $25, $17, $21, and $11)	938	707	2,565	1,441
Policy acquisition costs	2,178	1,970	6,142	5,415
Administrative expenses	1,060	883	2,959	2,479
Underwriting income (loss)	1,392	907	4,212	3,648
Net investment income (loss)	1,314	979	3,566	2,689
Other (income) expense	(154)	202	(550)	(9)
Amortization of purchased intangibles	84	69	226	211
Segment Income (loss)	2,776	1,615	8,102	6,135
Net realized gains (losses)	(103)	(456)	(484)	(936)
Market risk benefits gains (losses)	(32)	69	(154)	85
Interest Income (Expense), Net	174	150	499	416
Cigna integration expenses	14	23	51	26
Income tax expense	413	263	1,189	907
Net Income	2,040	792	5,725	3,935
Net loss attributable to noncontrolling interests	(3)	0	(3)	0
Net income (loss)	2,043	792	5,728	3,935
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	5,132	4,722	14,575	13,426
Net premiums earned	4,735	4,283	13,710	12,645
Losses and loss expenses	3,025	3,036	8,625	7,979
Policy benefits (includes remeasurement gains of $25, $17, $21, and $11)	0	0	0	0
Policy acquisition costs	640	583	1,867	1,701
Administrative expenses	323	272	934	814
Underwriting income (loss)	747	392	2,284	2,151
Net investment income (loss)	780	589	2,204	1,600
Other (income) expense	6	6	18	12
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	1,521	975	4,470	3,739
North American Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,527	1,392	4,404	3,998
Net premiums earned	1,407	1,334	4,084	3,852
Losses and loss expenses	900	857	2,634	2,343
Policy benefits (includes remeasurement gains of $25, $17, $21, and $11)	0	0	0	0
Policy acquisition costs	287	274	836	792
Administrative expenses	84	71	247	213
Underwriting income (loss)	136	132	367	504
Net investment income (loss)	94	76	262	199
Other (income) expense	2	1	2	3
Amortization of purchased intangibles	3	2	8	7
Segment Income (loss)	225	205	619	693
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,521	1,723	2,581	2,523
Net premiums earned	1,540	1,673	2,334	2,217
Losses and loss expenses	1,356	1,444	2,003	1,830
Policy benefits (includes remeasurement gains of $25, $17, $21, and $11)	0	0	0	0
Policy acquisition costs	76	68	128	111
Administrative expenses	3	3	9	4
Underwriting income (loss)	105	158	194	272
Net investment income (loss)	12	9	43	23
Other (income) expense	0	1	0	1
Amortization of purchased intangibles	6	7	19	20
Segment Income (loss)	111	159	218	274
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	3,211	2,645	9,359	8,364
Net premiums earned	3,311	2,741	9,005	8,065
Losses and loss expenses	1,635	1,333	4,139	3,772
Policy benefits (includes remeasurement gains of $25, $17, $21, and $11)	91	108	338	282
Policy acquisition costs	827	720	2,286	2,096
Administrative expenses	327	264	899	811
Underwriting income (loss)	431	316	1,343	1,104
Net investment income (loss)	248	151	636	460
Other (income) expense	(10)	(2)	(29)	3
Amortization of purchased intangibles	19	12	52	40
Segment Income (loss)	670	457	1,956	1,521
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	261	265	831	780
Net premiums earned	239	255	720	712
Losses and loss expenses	116	311	319	565
Policy benefits (includes remeasurement gains of $25, $17, $21, and $11)	0	0	0	0
Policy acquisition costs	69	59	196	178
Administrative expenses	9	8	27	27
Underwriting income (loss)	45	(123)	178	(58)
Net investment income (loss)	47	71	144	232
Other (income) expense	0	0	(1)	1
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	92	(52)	323	173
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,452	1,265	4,015	2,403
Net premiums earned	1,442	1,244	3,962	2,325
Losses and loss expenses	20	27	87	78
Policy benefits (includes remeasurement gains of $25, $17, $21, and $11)	866	666	2,283	1,275
Policy acquisition costs	279	266	829	537
Administrative expenses	216	174	553	346
Underwriting income (loss)	61	111	210	89
Net investment income (loss)	211	147	525	359
Other (income) expense	(28)	4	(69)	(38)
Amortization of purchased intangibles	12	2	18	7
Segment Income (loss)	288	252	786	479
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	61	74	133	275
Policy benefits (includes remeasurement gains of $25, $17, $21, and $11)	0	0	0	0
Policy acquisition costs	0	0	0	0
Administrative expenses	98	91	290	264
Underwriting income (loss)	(159)	(165)	(423)	(539)
Net investment income (loss)	14	5	28	(4)
Other (income) expense	(51)	194	(251)	73
Amortization of purchased intangibles	44	46	129	137
Segment Income (loss)	(138)	(400)	(273)	(753)
Net realized gains (losses)	(96)	(437)	(481)	(927)
Market risk benefits gains (losses)	(32)	69	(154)	85
Interest Income (Expense), Net	174	150	499	416
Cigna integration expenses	14	23	51	26
Income tax expense	413	263	1,189	907
Net Income	(867)	(1,204)	(2,647)	(2,944)
Net loss attributable to noncontrolling interests	(3)		(3)
Net income (loss)	(864)		(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	(7)	(19)	(3)	(9)
Policy benefits (includes remeasurement gains of $25, $17, $21, and $11)	(19)	(67)	(56)	(116)
Policy acquisition costs	0	0	0	0
Administrative expenses	0	0	0	0
Underwriting income (loss)	26	86	59	125
Net investment income (loss)	(92)	(69)	(276)	(180)
Other (income) expense	(73)	(2)	(220)	(64)
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	7	19	3	9
Net realized gains (losses)	(7)	(19)	(3)	(9)
Market risk benefits gains (losses)	0	0	0	0
Interest Income (Expense), Net	0	0	0	0
Cigna integration expenses	0	0	0	0
Income tax expense	0	0	0	0
Net Income	0	$ 0	0	$ 0
Net loss attributable to noncontrolling interests	0		0
Net income (loss)	$ 0		$ 0