Short Duration Contracts - Claims development - Insurance (Details) $ in Millions	Dec. 31, 2024 USD ($) claims	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance	$ 21,118
Insurance | Property, energy, marine and aviation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	3,272
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	2,177
All outstanding liabilities before 2015, net of reinsurance	19
Liabilities for losses and loss adjustment expenses, net of reinsurance	1,114
Insurance | Property, energy, marine and aviation | 2015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	89	$ 89	$ 91	$ 92	$ 92	$ 98	$ 102	$ 104	$ 110	$ 112
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	86	86	88	87	86	88	86	76	65	24
Total of IBNR liabilities plus expected development on reported claims	$ 1
Cumulative number of reported claims | claims	4,535
Insurance | Property, energy, marine and aviation | 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 86	87	87	92	96	100	105	101	104
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	86	87	87	91	94	97	98	83	25
Total of IBNR liabilities plus expected development on reported claims	$ 0
Cumulative number of reported claims | claims	6,177
Insurance | Property, energy, marine and aviation | 2017
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 224	225	225	231	230	236	246	281
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	221	220	218	216	212	196	140	30
Total of IBNR liabilities plus expected development on reported claims	$ 0
Cumulative number of reported claims | claims	6,481
Insurance | Property, energy, marine and aviation | 2018
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 170	172	170	170	174	186	181
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	157	154	150	143	135	102	30
Total of IBNR liabilities plus expected development on reported claims	$ 4
Cumulative number of reported claims | claims	5,085
Insurance | Property, energy, marine and aviation | 2019
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 156	159	161	165	179	179
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	153	148	139	134	105	26
Total of IBNR liabilities plus expected development on reported claims	$ (2)
Cumulative number of reported claims | claims	7,436
Insurance | Property, energy, marine and aviation | 2020
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 337	333	336	329	359
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	306	293	251	194	56
Total of IBNR liabilities plus expected development on reported claims	$ 3
Cumulative number of reported claims | claims	8,682
Insurance | Property, energy, marine and aviation | 2021
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 421	423	429	427
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	365	343	268	90
Total of IBNR liabilities plus expected development on reported claims	$ 19
Cumulative number of reported claims | claims	10,077
Insurance | Property, energy, marine and aviation | 2022
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 576	495	522
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	337	276	100
Total of IBNR liabilities plus expected development on reported claims	$ 148
Cumulative number of reported claims | claims	15,860
Insurance | Property, energy, marine and aviation | 2023
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 510	571
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	271	146
Total of IBNR liabilities plus expected development on reported claims	$ 110
Cumulative number of reported claims | claims	20,178
Insurance | Property, energy, marine and aviation | 2024
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 703
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	195
Total of IBNR liabilities plus expected development on reported claims	$ 348
Cumulative number of reported claims | claims	17,889
Insurance | Third party occurrence business
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 5,966
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	2,339
All outstanding liabilities before 2015, net of reinsurance	339
Liabilities for losses and loss adjustment expenses, net of reinsurance	3,966
Insurance | Third party occurrence business | 2015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	365	377	379	386	382	391	392	398	391	359
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	276	268	249	227	211	181	139	88	44	11
Total of IBNR liabilities plus expected development on reported claims	$ 51
Cumulative number of reported claims | claims	78,553
Insurance | Third party occurrence business | 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 345	352	363	367	375	399	406	394	389
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	246	230	215	195	164	137	88	42	12
Total of IBNR liabilities plus expected development on reported claims	$ 67
Cumulative number of reported claims | claims	78,811
Insurance | Third party occurrence business | 2017
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 408	404	406	407	412	422	418	417
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	271	247	221	165	135	100	52	13
Total of IBNR liabilities plus expected development on reported claims	$ 89
Cumulative number of reported claims | claims	84,682
Insurance | Third party occurrence business | 2018
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 448	461	459	451	450	453	430
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	271	247	200	154	115	64	17
Total of IBNR liabilities plus expected development on reported claims	$ 106
Cumulative number of reported claims | claims	79,202
Insurance | Third party occurrence business | 2019
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 451	470	471	480	487	456
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	255	214	173	122	73	18
Total of IBNR liabilities plus expected development on reported claims	$ 122
Cumulative number of reported claims | claims	88,964
Insurance | Third party occurrence business | 2020
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 606	632	640	616	606
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	318	235	155	76	24
Total of IBNR liabilities plus expected development on reported claims	$ 138
Cumulative number of reported claims | claims	98,530
Insurance | Third party occurrence business | 2021
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 671	659	662	622
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	323	174	91	26
Total of IBNR liabilities plus expected development on reported claims	$ 147
Cumulative number of reported claims | claims	99,287
Insurance | Third party occurrence business | 2022
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 735	726	688
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	186	85	24
Total of IBNR liabilities plus expected development on reported claims	$ 408
Cumulative number of reported claims | claims	100,455
Insurance | Third party occurrence business | 2023
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 936	877
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	156	32
Total of IBNR liabilities plus expected development on reported claims	$ 622
Cumulative number of reported claims | claims	102,558
Insurance | Third party occurrence business | 2024
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 1,001
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	37
Total of IBNR liabilities plus expected development on reported claims	$ 883
Cumulative number of reported claims | claims	83,386
Insurance | Third party claims-made business
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 4,745
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	2,302
All outstanding liabilities before 2015, net of reinsurance	90
Liabilities for losses and loss adjustment expenses, net of reinsurance	2,533
Insurance | Third party claims-made business | 2015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	268	274	267	268	252	255	260	276	277	259
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	248	242	221	217	193	174	126	100	52	9
Total of IBNR liabilities plus expected development on reported claims	$ 6
Cumulative number of reported claims | claims	13,955
Insurance | Third party claims-made business | 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 329	327	329	327	322	314	308	291	275
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	296	295	257	242	205	158	127	68	11
Total of IBNR liabilities plus expected development on reported claims	$ 16
Cumulative number of reported claims | claims	14,892
Insurance | Third party claims-made business | 2017
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 339	337	316	323	308	311	285	270
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	276	257	232	196	143	113	67	9
Total of IBNR liabilities plus expected development on reported claims	$ 42
Cumulative number of reported claims | claims	15,484
Insurance | Third party claims-made business | 2018
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 366	366	347	336	319	314	272
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	285	258	208	158	118	68	12
Total of IBNR liabilities plus expected development on reported claims	$ 33
Cumulative number of reported claims | claims	17,094
Insurance | Third party claims-made business | 2019
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 329	329	322	317	317	289
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	235	196	154	122	65	12
Total of IBNR liabilities plus expected development on reported claims	$ 49
Cumulative number of reported claims | claims	16,723
Insurance | Third party claims-made business | 2020
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 432	445	423	412	383
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	265	214	151	87	17
Total of IBNR liabilities plus expected development on reported claims	$ 86
Cumulative number of reported claims | claims	16,816
Insurance | Third party claims-made business | 2021
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 461	499	517	514
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	223	162	90	23
Total of IBNR liabilities plus expected development on reported claims	$ 176
Cumulative number of reported claims | claims	18,086
Insurance | Third party claims-made business | 2022
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 589	654	668
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	218	100	25
Total of IBNR liabilities plus expected development on reported claims	$ 257
Cumulative number of reported claims | claims	19,825
Insurance | Third party claims-made business | 2023
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 895	809
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	200	64
Total of IBNR liabilities plus expected development on reported claims	$ 543
Cumulative number of reported claims | claims	23,671
Insurance | Third party claims-made business | 2024
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 737
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	56
Total of IBNR liabilities plus expected development on reported claims	$ 577
Cumulative number of reported claims | claims	23,155
Insurance | Multi-line and other specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 6,518
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	4,606
All outstanding liabilities before 2015, net of reinsurance	29
Liabilities for losses and loss adjustment expenses, net of reinsurance	1,941
Insurance | Multi-line and other specialty | 2015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	344	344	345	347	349	356	365	356	358	334
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	337	333	331	330	326	321	306	278	236	$ 138
Total of IBNR liabilities plus expected development on reported claims	$ 3
Cumulative number of reported claims | claims	181,424
Insurance | Multi-line and other specialty | 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 404	406	408	408	410	416	427	431	408
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	396	391	390	385	379	363	341	304	$ 176
Total of IBNR liabilities plus expected development on reported claims	$ 4
Cumulative number of reported claims | claims	195,233
Insurance | Multi-line and other specialty | 2017
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 514	516	512	504	500	491	500	482
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	493	479	472	446	423	380	342	$ 181
Total of IBNR liabilities plus expected development on reported claims	$ 6
Cumulative number of reported claims | claims	236,433
Insurance | Multi-line and other specialty | 2018
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 564	564	564	564	562	564	512
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	543	526	508	479	442	389	$ 211
Total of IBNR liabilities plus expected development on reported claims	$ 9
Cumulative number of reported claims | claims	266,125
Insurance | Multi-line and other specialty | 2019
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 671	656	650	639	611	566
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	611	576	548	486	385	$ 212
Total of IBNR liabilities plus expected development on reported claims	$ 11
Cumulative number of reported claims | claims	249,876
Insurance | Multi-line and other specialty | 2020
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 519	515	513	567	616
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	450	405	358	308	$ 171
Total of IBNR liabilities plus expected development on reported claims	$ 26
Cumulative number of reported claims | claims	168,232
Insurance | Multi-line and other specialty | 2021
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 635	614	618	634
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	511	427	334	$ 157
Total of IBNR liabilities plus expected development on reported claims	$ 42
Cumulative number of reported claims | claims	112,591
Insurance | Multi-line and other specialty | 2022
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 639	640	677
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	439	370	$ 177
Total of IBNR liabilities plus expected development on reported claims	$ 90
Cumulative number of reported claims | claims	144,429
Insurance | Multi-line and other specialty | 2023
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 809	815
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	489	$ 253
Total of IBNR liabilities plus expected development on reported claims	$ 199
Cumulative number of reported claims | claims	161,842
Insurance | Multi-line and other specialty | 2024
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 1,419
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	337
Total of IBNR liabilities plus expected development on reported claims	$ 885
Cumulative number of reported claims | claims	145,190