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			60 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2021	Dec. 31, 2018
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss adjustment expenses	$ 39,659	$ 37,855		$ 39,659
Asbestos and Environmental
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid for Accident Years not Displayed in Triangles	604			604
Policyholder Benefits and Claims Incurred, Ceded		220		1,015
Adverse Development Cover Navigators Group [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	300
Deferred Revenue	209			209
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	724			724
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	0			0
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	724			724
Reinsurance and Other Recoverables	1,545			1,545
Unpaid losses and loss adjustment expenses	2,269			2,269
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	18,263			18,263
Cumulative Paid for Accident Years Displayed in Triangles	(9,992)			(9,992)
Unpaid for Accident Years not Displayed in Triangles	2,981			2,981
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	348			348
Amount of discount	341			341
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	11,259			11,259
Reinsurance and Other Recoverables	1,793			1,793
Unpaid losses and loss adjustment expenses	13,052			13,052
General liability | Adverse Development Cover Navigators Group [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	86
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	6,731			6,731
Cumulative Paid for Accident Years Displayed in Triangles	(3,157)			(3,157)
Unpaid for Accident Years not Displayed in Triangles	1,240			1,240
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	146			146
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	4,960			4,960
Reinsurance and Other Recoverables	870			870
Unpaid losses and loss adjustment expenses	5,830			5,830
Marine [Member] | Adverse Development Cover Navigators Group [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	39
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	1,516			1,516
Cumulative Paid for Accident Years Displayed in Triangles	(1,242)			(1,242)
Unpaid for Accident Years not Displayed in Triangles	17			17
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	12			12
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	303			303
Reinsurance and Other Recoverables	235			235
Unpaid losses and loss adjustment expenses	538			538
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	6,952			6,952
Cumulative Paid for Accident Years Displayed in Triangles	(5,208)			(5,208)
Unpaid for Accident Years not Displayed in Triangles	77			77
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	103			103
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,924			1,924
Reinsurance and Other Recoverables	90			90
Unpaid losses and loss adjustment expenses	2,014			2,014
Property | P&C Commercial Lines | Adverse Development Cover Navigators Group [Member]
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	3,830			3,830
Cumulative Paid for Accident Years Displayed in Triangles	(3,342)			(3,342)
Unpaid for Accident Years not Displayed in Triangles	24			24
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	18			18
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	530			530
Reinsurance and Other Recoverables	268			268
Unpaid losses and loss adjustment expenses	798			798
Automobile liability | P&C Commercial Lines | Adverse Development Cover Navigators Group [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	16
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	3,908			3,908
Cumulative Paid for Accident Years Displayed in Triangles	(2,778)			(2,778)
Unpaid for Accident Years not Displayed in Triangles	22			22
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	23			23
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,175			1,175
Reinsurance and Other Recoverables	91			91
Unpaid losses and loss adjustment expenses	1,266			1,266
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	11,104			11,104
Cumulative Paid for Accident Years Displayed in Triangles	(9,809)			(9,809)
Unpaid for Accident Years not Displayed in Triangles	29			29
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	66			66
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,390			1,390
Reinsurance and Other Recoverables	23			23
Unpaid losses and loss adjustment expenses	1,413			1,413
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	172			172
Cumulative Paid for Accident Years Displayed in Triangles	(162)			(162)
Unpaid for Accident Years not Displayed in Triangles	4			4
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	0			0
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	14			14
Reinsurance and Other Recoverables	(1)			(1)
Unpaid losses and loss adjustment expenses	13			13
Professional liability | Adverse Development Cover Navigators Group [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	110
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	2,511			2,511
Cumulative Paid for Accident Years Displayed in Triangles	(1,335)			(1,335)
Unpaid for Accident Years not Displayed in Triangles	47			47
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	38			38
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,261			1,261
Reinsurance and Other Recoverables	781			781
Unpaid losses and loss adjustment expenses	2,042			2,042
Bond | Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	625			625
Cumulative Paid for Accident Years Displayed in Triangles	(254)			(254)
Unpaid for Accident Years not Displayed in Triangles	32			32
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	31			31
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	434			434
Reinsurance and Other Recoverables	10			10
Unpaid losses and loss adjustment expenses	444			444
Assumed Reinsurance [Member] | Adverse Development Cover Navigators Group [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	29
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	1,384			1,384
Cumulative Paid for Accident Years Displayed in Triangles	(1,106)			(1,106)
Unpaid for Accident Years not Displayed in Triangles	3			3
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	4			4
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	285			285
Reinsurance and Other Recoverables	50			50
Unpaid losses and loss adjustment expenses	335			335
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	1,200			1,200
Cumulative Paid for Accident Years Displayed in Triangles	(1,170)			(1,170)
Unpaid for Accident Years not Displayed in Triangles	7			7
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	3			3
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	40			40
Reinsurance and Other Recoverables	0			0
Unpaid losses and loss adjustment expenses	40			40
Homeowners | Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	6,307			6,307
Cumulative Paid for Accident Years Displayed in Triangles	(5,982)			(5,982)
Unpaid for Accident Years not Displayed in Triangles	5			5
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	34			34
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	364			364
Reinsurance and Other Recoverables	17			17
Unpaid losses and loss adjustment expenses	381			381
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	(5,982)			(5,982)
Other ongoing business | Adverse Development Cover Navigators Group [Member]
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	183			183
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	5			5
Amount of discount	14			14
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	174			174
Reinsurance and Other Recoverables	310			310
Unpaid losses and loss adjustment expenses	484			484
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	374			374
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	157			157
Amount of discount	0			0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	531			531
Reinsurance and Other Recoverables	(1)			(1)
Unpaid losses and loss adjustment expenses	530			530
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	64,503			64,503
Cumulative Paid for Accident Years Displayed in Triangles	(45,537)			(45,537)
Unpaid for Accident Years not Displayed in Triangles	5,769			5,769
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	988			988
Amount of discount	355	367	$ 388	355
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	25,368	23,897	22,986	25,368	$ 20,352
Reinsurance and Other Recoverables	6,081	5,725	5,275	6,081	4,232
Unpaid losses and loss adjustment expenses	31,449	29,622	28,261	$ 31,449	$ 24,584
Policyholder Benefits and Claims Incurred, Ceded	$ 1,243	$ 1,156	$ 826