Reserve for Unpaid Losses and Loss Adjustment Expenses - Reconciliation of Loss Development to Liability for Unpaid Losses and Loss Adjustment Expenses, P&C (Details) - USD ($)
$ in Millions

Reserve for Unpaid Losses and Loss Adjustment Expenses - Reconciliation of Loss Development to Liability for Unpaid Losses and Loss Adjustment Expenses, P&C (Details) - USD ($) $ in Millions	12 Months Ended			84 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2023	Dec. 31, 2020
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss adjustment expenses	$ 42,318	$ 41,243		$ 42,318
Asbestos and Environmental
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid for Accident Years not Displayed in Triangles	248			248
Policyholder Benefits and Claims Incurred, Ceded				1,438
Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	300
Deferred Revenue	209			209
Reinsurance Policy, Type [Axis]: Asbestos and Environmental \| Asbestos and Environmental
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	1,438
Deferred Revenue	788			788
Segment Reconciling Items \| Asbestos and Environmental
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid for Accident Years not Displayed in Triangles	340			340
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	0			0
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	340			340
Reinsurance and Other Recoverables	1,992			1,992
Unpaid losses and loss adjustment expenses	2,332			2,332
Workers’ compensation \| Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	18,050			18,050
Cumulative Paid for Accident Years Displayed in Triangles	(9,482)			(9,482)
Unpaid for Accident Years not Displayed in Triangles	3,581			3,581
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	388			388
Amount of discount	325			325
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	12,212			12,212
Reinsurance and Other Recoverables	1,651			1,651
Unpaid losses and loss adjustment expenses	13,863			13,863
General liability \| Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	105
General liability \| Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	8,271			8,271
Cumulative Paid for Accident Years Displayed in Triangles	(3,772)			(3,772)
Unpaid for Accident Years not Displayed in Triangles	497			497
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	172			172
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	5,168			5,168
Reinsurance and Other Recoverables	1,160			1,160
Unpaid losses and loss adjustment expenses	6,328			6,328
Marine [Member] \| Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	38
Marine [Member] \| Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,452			1,452
Cumulative Paid for Accident Years Displayed in Triangles	(1,182)			(1,182)
Unpaid for Accident Years not Displayed in Triangles	11			11
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	14			14
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	295			295
Reinsurance and Other Recoverables	255			255
Unpaid losses and loss adjustment expenses	550			550
Package business \| Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	7,856			7,856
Cumulative Paid for Accident Years Displayed in Triangles	(5,797)			(5,797)
Unpaid for Accident Years not Displayed in Triangles	102			102
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	120			120
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	2,281			2,281
Reinsurance and Other Recoverables	54			54
Unpaid losses and loss adjustment expenses	2,335			2,335
Property \| P&C Commercial Lines \| Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	3
Property \| P&C Commercial Lines \| Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	4,068			4,068
Cumulative Paid for Accident Years Displayed in Triangles	(3,568)			(3,568)
Unpaid for Accident Years not Displayed in Triangles	13			13
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	27			27
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	540			540
Reinsurance and Other Recoverables	271			271
Unpaid losses and loss adjustment expenses	811			811
Automobile liability \| P&C Commercial Lines \| Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	14
Automobile liability \| P&C Commercial Lines \| Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	4,243			4,243
Cumulative Paid for Accident Years Displayed in Triangles	(2,945)			(2,945)
Unpaid for Accident Years not Displayed in Triangles	21			21
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	32			32
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,351			1,351
Reinsurance and Other Recoverables	99			99
Unpaid losses and loss adjustment expenses	1,450			1,450
Automobile liability \| P&C Personal Lines \| Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	10,874			10,874
Cumulative Paid for Accident Years Displayed in Triangles	(9,393)			(9,393)
Unpaid for Accident Years not Displayed in Triangles	29			29
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	62			62
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,572			1,572
Reinsurance and Other Recoverables	27			27
Unpaid losses and loss adjustment expenses	1,599			1,599
Commercial automobile physical damage \| P&C Commercial Lines \| Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	215			215
Cumulative Paid for Accident Years Displayed in Triangles	(194)			(194)
Unpaid for Accident Years not Displayed in Triangles	3			3
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	1			1
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	25			25
Reinsurance and Other Recoverables	0			0
Unpaid losses and loss adjustment expenses	25			25
Professional liability \| Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	95
Professional liability \| Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	2,846			2,846
Cumulative Paid for Accident Years Displayed in Triangles	(1,511)			(1,511)
Unpaid for Accident Years not Displayed in Triangles	68			68
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	46			46
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,449			1,449
Reinsurance and Other Recoverables	734			734
Unpaid losses and loss adjustment expenses	2,183			2,183
Bond \| Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	1
Bond \| Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	680			680
Cumulative Paid for Accident Years Displayed in Triangles	(284)			(284)
Unpaid for Accident Years not Displayed in Triangles	26			26
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	36			36
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	458			458
Reinsurance and Other Recoverables	14			14
Unpaid losses and loss adjustment expenses	472			472
Assumed Reinsurance [Member] \| Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	27
Assumed Reinsurance [Member] \| Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,798			1,798
Cumulative Paid for Accident Years Displayed in Triangles	(1,242)			(1,242)
Unpaid for Accident Years not Displayed in Triangles	0			0
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	6			6
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	562			562
Reinsurance and Other Recoverables	44			44
Unpaid losses and loss adjustment expenses	606			606
Personal automobile physical damage \| Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,535			1,535
Cumulative Paid for Accident Years Displayed in Triangles	(1,462)			(1,462)
Unpaid for Accident Years not Displayed in Triangles	7			7
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	4			4
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	84			84
Reinsurance and Other Recoverables	0			0
Unpaid losses and loss adjustment expenses	84			84
Homeowners \| Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	6,009			6,009
Cumulative Paid for Accident Years Displayed in Triangles	(5,677)			(5,677)
Unpaid for Accident Years not Displayed in Triangles	5			5
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	35			35
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	372			372
Reinsurance and Other Recoverables	4			4
Unpaid losses and loss adjustment expenses	376			376
Homeowners \| P&C Personal Lines \| Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Paid for Accident Years Displayed in Triangles	(5,677)			(5,677)
Short-Duration Insurance, Other [Member] \| Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	17
Short-Duration Insurance, Other [Member] \| Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid for Accident Years not Displayed in Triangles	170			170
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	5			5
Amount of discount	14			14
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	161			161
Reinsurance and Other Recoverables	347			347
Unpaid losses and loss adjustment expenses	508			508
Other operations \| Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid for Accident Years not Displayed in Triangles	317			317
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	161			161
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	478			478
Reinsurance and Other Recoverables	44			44
Unpaid losses and loss adjustment expenses	522			522
Property and Casualty Insurance Products
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	67,897			67,897
Cumulative Paid for Accident Years Displayed in Triangles	(46,509)			(46,509)
Unpaid for Accident Years not Displayed in Triangles	5,190			5,190
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	1,109			1,109
Amount of discount	339	347	$ 355	339
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	27,348	26,618	25,368	27,348	$ 23,897
Reinsurance and Other Recoverables	6,696	6,465	6,081	6,696	5,725
Unpaid losses and loss adjustment expenses	34,044	33,083	31,449	$ 34,044	$ 29,622
Policyholder Benefits and Claims Incurred, Ceded	$ 1,043	$ 1,338	$ 1,243