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			72 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2022	Dec. 31, 2019
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss adjustment expenses	$ 41,243	$ 39,659		$ 41,243
Asbestos and Environmental
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid for Accident Years not Displayed in Triangles	390			390
Policyholder Benefits and Claims Incurred, Ceded			$ 220	1,244
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
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Policyholder Benefits and Claims Incurred, Ceded			220
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	1,244
Deferred Revenue	594			594
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	495			495
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	0			0
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	495			495
Reinsurance and Other Recoverables	1,789			1,789
Unpaid losses and loss adjustment expenses	2,284			2,284
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	17,911			17,911
Cumulative Paid for Accident Years Displayed in Triangles	(9,516)			(9,516)
Unpaid for Accident Years not Displayed in Triangles	3,295			3,295
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	372			372
Amount of discount	333			333
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	11,729			11,729
Reinsurance and Other Recoverables	1,648			1,648
Unpaid losses and loss adjustment expenses	13,377			13,377
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	7,509			7,509
Cumulative Paid for Accident Years Displayed in Triangles	(3,486)			(3,486)
Unpaid for Accident Years not Displayed in Triangles	1,238			1,238
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	153			153
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	5,414			5,414
Reinsurance and Other Recoverables	1,070			1,070
Unpaid losses and loss adjustment expenses	6,484			6,484
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	1,476			1,476
Cumulative Paid for Accident Years Displayed in Triangles	(1,192)			(1,192)
Unpaid for Accident Years not Displayed in Triangles	14			14
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	14			14
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	312			312
Reinsurance and Other Recoverables	266			266
Unpaid losses and loss adjustment expenses	578			578
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	7,218			7,218
Cumulative Paid for Accident Years Displayed in Triangles	(5,288)			(5,288)
Unpaid for Accident Years not Displayed in Triangles	87			87
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	112			112
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	2,129			2,129
Reinsurance and Other Recoverables	72			72
Unpaid losses and loss adjustment expenses	2,201			2,201
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	3,922			3,922
Cumulative Paid for Accident Years Displayed in Triangles	(3,383)			(3,383)
Unpaid for Accident Years not Displayed in Triangles	26			26
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	20			20
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	585			585
Reinsurance and Other Recoverables	329			329
Unpaid losses and loss adjustment expenses	914			914
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	4,025			4,025
Cumulative Paid for Accident Years Displayed in Triangles	(2,824)			(2,824)
Unpaid for Accident Years not Displayed in Triangles	22			22
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	26			26
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,249			1,249
Reinsurance and Other Recoverables	88			88
Unpaid losses and loss adjustment expenses	1,337			1,337
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	10,887			10,887
Cumulative Paid for Accident Years Displayed in Triangles	(9,584)			(9,584)
Unpaid for Accident Years not Displayed in Triangles	31			31
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	60			60
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,394			1,394
Reinsurance and Other Recoverables	23			23
Unpaid losses and loss adjustment expenses	1,417			1,417
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	181			181
Cumulative Paid for Accident Years Displayed in Triangles	(169)			(169)
Unpaid for Accident Years not Displayed in Triangles	4			4
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	1			1
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	17			17
Reinsurance and Other Recoverables	(1)			(1)
Unpaid losses and loss adjustment expenses	16			16
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	2,609			2,609
Cumulative Paid for Accident Years Displayed in Triangles	(1,403)			(1,403)
Unpaid for Accident Years not Displayed in Triangles	67			67
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	43			43
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,316			1,316
Reinsurance and Other Recoverables	734			734
Unpaid losses and loss adjustment expenses	2,050			2,050
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	635			635
Cumulative Paid for Accident Years Displayed in Triangles	(260)			(260)
Unpaid for Accident Years not Displayed in Triangles	43			43
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	34			34
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	452			452
Reinsurance and Other Recoverables	15			15
Unpaid losses and loss adjustment expenses	467			467
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	1,541			1,541
Cumulative Paid for Accident Years Displayed in Triangles	(1,133)			(1,133)
Unpaid for Accident Years not Displayed in Triangles	2			2
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	4			4
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	414			414
Reinsurance and Other Recoverables	50			50
Unpaid losses and loss adjustment expenses	464			464
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	1,288			1,288
Cumulative Paid for Accident Years Displayed in Triangles	(1,250)			(1,250)
Unpaid for Accident Years not Displayed in Triangles	6			6
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	4			4
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	48			48
Reinsurance and Other Recoverables	0			0
Unpaid losses and loss adjustment expenses	48			48
Homeowners \| Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	6,063			6,063
Cumulative Paid for Accident Years Displayed in Triangles	(5,728)			(5,728)
Unpaid for Accident Years not Displayed in Triangles	3			3
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	36			36
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	374			374
Reinsurance and Other Recoverables	8			8
Unpaid losses and loss adjustment expenses	382			382
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	(5,728)			(5,728)
Other ongoing business \| 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
Other ongoing business \| Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid for Accident Years not Displayed in Triangles	178			178
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	6			6
Amount of discount	14			14
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	170			170
Reinsurance and Other Recoverables	333			333
Unpaid losses and loss adjustment expenses	503			503
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	362			362
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	158			158
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	520			520
Reinsurance and Other Recoverables	41			41
Unpaid losses and loss adjustment expenses	561			561
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	65,265			65,265
Cumulative Paid for Accident Years Displayed in Triangles	(45,216)			(45,216)
Unpaid for Accident Years not Displayed in Triangles	5,873			5,873
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	1,043			1,043
Amount of discount	347	355	367	347
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	26,618	25,368	23,897	26,618	$ 22,986
Reinsurance and Other Recoverables	6,465	6,081	5,725	6,465	5,275
Unpaid losses and loss adjustment expenses	33,083	31,449	29,622	$ 33,083	$ 28,261
Policyholder Benefits and Claims Incurred, Ceded	$ 1,338	$ 1,243	$ 1,156