Short Duration Contracts - Claims development - Insurance (Details) $ in Millions	Dec. 31, 2023 USD ($) claims	Dec. 31, 2022 USD ($)	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 ($)	Dec. 31, 2014 USD ($)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance	$ 15,700
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	2,688
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	1,878
All outstanding liabilities before 2014, net of reinsurance	14
Liabilities for losses and loss adjustment expenses, net of reinsurance	824
Insurance | Property, energy, marine and aviation | 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	133	$ 134	$ 135	$ 135	$ 134	$ 132	$ 136	$ 147	$ 146	$ 148
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	125	123	122	115	98	88	84	78	54	26
Total of IBNR liabilities plus expected development on reported claims	$ 2
Cumulative number of reported claims | claims	3,871
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	91	92	92	98	102	104	110	112
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	86	88	87	86	88	86	76	65	24
Total of IBNR liabilities plus expected development on reported claims	$ 2
Cumulative number of reported claims | claims	4,530
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	$ 88	88	92	96	100	105	101	104
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	87	87	91	95	97	98	83	25
Total of IBNR liabilities plus expected development on reported claims	$ 0
Cumulative number of reported claims | claims	6,171
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	$ 225	225	231	230	236	246	281
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	220	218	216	212	196	140	30
Total of IBNR liabilities plus expected development on reported claims	$ 1
Cumulative number of reported claims | claims	6,475
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	$ 172	170	170	174	186	181
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	154	150	143	135	102	30
Total of IBNR liabilities plus expected development on reported claims	$ 5
Cumulative number of reported claims | claims	5,077
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	$ 159	161	165	179	179
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	148	139	134	105	26
Total of IBNR liabilities plus expected development on reported claims	$ (1)
Cumulative number of reported claims | claims	7,018
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	$ 333	336	329	359
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	293	251	194	56
Total of IBNR liabilities plus expected development on reported claims	$ 9
Cumulative number of reported claims | claims	8,212
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	$ 423	429	427
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	343	268	90
Total of IBNR liabilities plus expected development on reported claims	$ 41
Cumulative number of reported claims | claims	8,807
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	$ 495	522
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	276	100
Total of IBNR liabilities plus expected development on reported claims	$ 74
Cumulative number of reported claims | claims	13,274
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	$ 571
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	146
Total of IBNR liabilities plus expected development on reported claims	$ 275
Cumulative number of reported claims | claims	13,682
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	$ 5,304
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	1,990
All outstanding liabilities before 2014, net of reinsurance	287
Liabilities for losses and loss adjustment expenses, net of reinsurance	3,601
Insurance | Third party occurrence business | 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	344	345	343	343	344	339	343	339	336	330
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	257	237	224	211	191	162	113	71	40	9
Total of IBNR liabilities plus expected development on reported claims	$ 49
Cumulative number of reported claims | claims	75,679
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	$ 377	379	386	382	391	392	399	392	359
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	268	249	227	212	182	139	88	45	11
Total of IBNR liabilities plus expected development on reported claims	$ 66
Cumulative number of reported claims | claims	78,689
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	$ 352	363	368	375	399	406	394	390
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	231	215	195	165	137	88	42	12
Total of IBNR liabilities plus expected development on reported claims	$ 77
Cumulative number of reported claims | claims	78,883
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	$ 405	406	407	412	422	418	417
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	247	221	165	135	100	52	13
Total of IBNR liabilities plus expected development on reported claims	$ 100
Cumulative number of reported claims | claims	84,687
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	$ 462	459	451	451	453	430
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	247	200	154	115	64	17
Total of IBNR liabilities plus expected development on reported claims	$ 127
Cumulative number of reported claims | claims	78,320
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	$ 470	471	481	487	456
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	214	173	122	73	18
Total of IBNR liabilities plus expected development on reported claims	$ 154
Cumulative number of reported claims | claims	86,139
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	$ 632	640	616	607
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	235	155	77	24
Total of IBNR liabilities plus expected development on reported claims	$ 226
Cumulative number of reported claims | claims	92,279
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	$ 659	663	622
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	174	91	26
Total of IBNR liabilities plus expected development on reported claims	$ 290
Cumulative number of reported claims | claims	92,672
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	$ 726	688
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	85	24
Total of IBNR liabilities plus expected development on reported claims	$ 513
Cumulative number of reported claims | claims	92,564
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	$ 877
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	32
Total of IBNR liabilities plus expected development on reported claims	$ 750
Cumulative number of reported claims | claims	72,833
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	$ 4,334
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	2,059
All outstanding liabilities before 2014, net of reinsurance	77
Liabilities for losses and loss adjustment expenses, net of reinsurance	2,352
Insurance | Third party claims-made business | 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	287	296	288	291	297	281	279	299	279	264
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	266	260	249	243	229	207	173	129	63	14
Total of IBNR liabilities plus expected development on reported claims	$ 14
Cumulative number of reported claims | claims	14,494
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	$ 274	267	268	252	255	260	276	277	259
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	242	221	217	193	174	126	100	52	9
Total of IBNR liabilities plus expected development on reported claims	$ 13
Cumulative number of reported claims | claims	13,932
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	$ 327	330	327	322	314	308	291	275
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	296	257	242	205	158	127	68	11
Total of IBNR liabilities plus expected development on reported claims	$ 18
Cumulative number of reported claims | claims	14,912
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	$ 338	317	323	308	312	286	270
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	258	233	196	143	113	68	9
Total of IBNR liabilities plus expected development on reported claims	$ 48
Cumulative number of reported claims | claims	15,473
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	$ 367	347	336	320	314	273
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	258	208	158	118	68	12
Total of IBNR liabilities plus expected development on reported claims	$ 44
Cumulative number of reported claims | claims	17,098
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	$ 330	322	317	317	289
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	197	155	122	65	12
Total of IBNR liabilities plus expected development on reported claims	$ 66
Cumulative number of reported claims | claims	15,879
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	$ 446	423	413	383
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	215	152	87	17
Total of IBNR liabilities plus expected development on reported claims	$ 118
Cumulative number of reported claims | claims	15,239
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	$ 499	518	515
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	163	90	23
Total of IBNR liabilities plus expected development on reported claims	$ 254
Cumulative number of reported claims | claims	16,323
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	$ 655	669
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	100	25
Total of IBNR liabilities plus expected development on reported claims	$ 448
Cumulative number of reported claims | claims	18,563
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	$ 811
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	64
Total of IBNR liabilities plus expected development on reported claims	$ 641
Cumulative number of reported claims | claims	20,494
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	$ 5,389
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	4,065
All outstanding liabilities before 2014, net of reinsurance	24
Liabilities for losses and loss adjustment expenses, net of reinsurance	1,348
Insurance | Multi-line and other specialty | 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	310	311	309	310	313	317	318	318	326	302
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	297	296	295	293	292	281	267	234	197	$ 108
Total of IBNR liabilities plus expected development on reported claims	$ 4
Cumulative number of reported claims | claims	148,702
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	$ 345	345	347	349	357	365	357	358	335
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	333	331	330	327	321	306	278	236	$ 138
Total of IBNR liabilities plus expected development on reported claims	$ 5
Cumulative number of reported claims | claims	180,468
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	$ 406	409	408	410	416	428	431	409
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	392	390	385	379	363	342	305	$ 176
Total of IBNR liabilities plus expected development on reported claims	$ 7
Cumulative number of reported claims | claims	192,955
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	$ 516	513	504	501	491	501	482
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	480	472	446	423	381	342	$ 181
Total of IBNR liabilities plus expected development on reported claims	$ 9
Cumulative number of reported claims | claims	235,539
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	$ 565	565	565	563	565	512
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	527	509	480	443	389	$ 212
Total of IBNR liabilities plus expected development on reported claims	$ 14
Cumulative number of reported claims | claims	260,995
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	$ 657	651	640	612	567
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	577	549	487	386	$ 212
Total of IBNR liabilities plus expected development on reported claims	$ 21
Cumulative number of reported claims | claims	252,766
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	$ 517	515	569	618
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	406	359	309	$ 172
Total of IBNR liabilities plus expected development on reported claims	$ 44
Cumulative number of reported claims | claims	166,357
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	$ 614	619	635
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	428	335	$ 157
Total of IBNR liabilities plus expected development on reported claims	$ 76
Cumulative number of reported claims | claims	114,447
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	$ 642	678
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	371	$ 177
Total of IBNR liabilities plus expected development on reported claims	$ 157
Cumulative number of reported claims | claims	143,264
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	$ 817
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	254
Total of IBNR liabilities plus expected development on reported claims	$ 450
Cumulative number of reported claims | claims	118,241