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			19 Months Ended	48 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2017
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss adjustment expenses	$ 37,855	$ 36,517		$ 37,855	$ 37,855
Asbestos and Environmental
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Policyholder Benefits and Claims Incurred, Ceded				220	860
Adverse Development Cover Navigators Group [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	209
Deferred Revenue	118			118	118
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	938			938	938
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	0			0	0
Discount	0			0	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	938			938	938
Reinsurance and Other Recoverables	1,429			1,429	1,429
Unpaid losses and loss adjustment expenses	2,367			2,367	2,367
Workers’ compensation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Paid for Accident Years Displayed in Triangles	(10,633)			(10,633)	(10,633)
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	18,864			18,864	18,864
Cumulative Paid for Accident Years Displayed in Triangles	(10,633)			(10,633)	(10,633)
Unpaid for Accident Years not Displayed in Triangles	2,671			2,671	2,671
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	336			336	336
Discount	(352)			(352)	(352)
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	10,886			10,886	10,886
Reinsurance and Other Recoverables	1,970			1,970	1,970
Unpaid losses and loss adjustment expenses	12,856			12,856	12,856
General liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Paid for Accident Years Displayed in Triangles	(2,946)			(2,946)	(2,946)
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	53
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	6,201			6,201	6,201
Cumulative Paid for Accident Years Displayed in Triangles	(2,946)			(2,946)	(2,946)
Unpaid for Accident Years not Displayed in Triangles	713			713	713
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	137			137	137
Discount	0			0	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	4,105			4,105	4,105
Reinsurance and Other Recoverables	740			740	740
Unpaid losses and loss adjustment expenses	4,845			4,845	4,845
Marine [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,379			1,379	1,379
Cumulative Paid for Accident Years Displayed in Triangles	(1,128)			(1,128)	(1,128)
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	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	1,379			1,379	1,379
Cumulative Paid for Accident Years Displayed in Triangles	(1,128)			(1,128)	(1,128)
Unpaid for Accident Years not Displayed in Triangles	18			18	18
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	10			10	10
Discount	0			0	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	279			279	279
Reinsurance and Other Recoverables	216			216	216
Unpaid losses and loss adjustment expenses	495			495	495
Package business
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Paid for Accident Years Displayed in Triangles	(5,248)			(5,248)	(5,248)
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	6,940			6,940	6,940
Cumulative Paid for Accident Years Displayed in Triangles	(5,248)			(5,248)	(5,248)
Unpaid for Accident Years not Displayed in Triangles	66			66	66
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	94			94	94
Discount	0			0	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,852			1,852	1,852
Reinsurance and Other Recoverables	17			17	17
Unpaid losses and loss adjustment expenses	1,869			1,869	1,869
Property | P&C Commercial Lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Paid for Accident Years Displayed in Triangles	(2,936)			(2,936)	(2,936)
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	5
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	3,365			3,365	3,365
Cumulative Paid for Accident Years Displayed in Triangles	(2,936)			(2,936)	(2,936)
Unpaid for Accident Years not Displayed in Triangles	28			28	28
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	18			18	18
Discount	0			0	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	475			475	475
Reinsurance and Other Recoverables	221			221	221
Unpaid losses and loss adjustment expenses	696			696	696
Automobile liability | P&C Commercial Lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Paid for Accident Years Displayed in Triangles	(2,775)			(2,775)	(2,775)
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	12
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	3,804			3,804	3,804
Cumulative Paid for Accident Years Displayed in Triangles	(2,775)			(2,775)	(2,775)
Unpaid for Accident Years not Displayed in Triangles	16			16	16
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	21			21	21
Discount	0			0	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,066			1,066	1,066
Reinsurance and Other Recoverables	78			78	78
Unpaid losses and loss adjustment expenses	1,144			1,144	1,144
Automobile liability | P&C Personal Lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Paid for Accident Years Displayed in Triangles	(10,153)			(10,153)	(10,153)
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	11,463			11,463	11,463
Cumulative Paid for Accident Years Displayed in Triangles	(10,153)			(10,153)	(10,153)
Unpaid for Accident Years not Displayed in Triangles	29			29	29
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	60			60	60
Discount	0			0	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,399			1,399	1,399
Reinsurance and Other Recoverables	28			28	28
Unpaid losses and loss adjustment expenses	1,427			1,427	1,427
Commercial automobile physical damage
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Paid for Accident Years Displayed in Triangles	(167)			(167)	(167)
Commercial 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	176			176	176
Cumulative Paid for Accident Years Displayed in Triangles	(167)			(167)	(167)
Unpaid for Accident Years not Displayed in Triangles	4			4	4
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	0			0	0
Discount	0			0	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	13			13	13
Reinsurance and Other Recoverables	(1)			(1)	(1)
Unpaid losses and loss adjustment expenses	12			12	12
Professional liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Paid for Accident Years Displayed in Triangles	(1,080)			(1,080)	(1,080)
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	53
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	2,164			2,164	2,164
Cumulative Paid for Accident Years Displayed in Triangles	(1,080)			(1,080)	(1,080)
Unpaid for Accident Years not Displayed in Triangles	65			65	65
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	35			35	35
Discount	0			0	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,184			1,184	1,184
Reinsurance and Other Recoverables	689			689	689
Unpaid losses and loss adjustment expenses	1,873			1,873	1,873
Bond
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Paid for Accident Years Displayed in Triangles	(310)			(310)	(310)
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	635
Cumulative Paid for Accident Years Displayed in Triangles	(310)			(310)	(310)
Unpaid for Accident Years not Displayed in Triangles	28			28	28
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	28			28	28
Discount	0			0	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	381			381	381
Reinsurance and Other Recoverables	13			13	13
Unpaid losses and loss adjustment expenses	394			394	394
Assumed Reinsurance [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,198			1,198	1,198
Cumulative Paid for Accident Years Displayed in Triangles	(986)			(986)	(986)
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	24
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	1,198			1,198	1,198
Cumulative Paid for Accident Years Displayed in Triangles	(986)			(986)	(986)
Unpaid for Accident Years not Displayed in Triangles	3			3	3
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	3			3	3
Discount	0			0	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	218			218	218
Reinsurance and Other Recoverables	41			41	41
Unpaid losses and loss adjustment expenses	259			259	259
Personal automobile physical damage
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Paid for Accident Years Displayed in Triangles	(1,262)			(1,262)	(1,262)
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	1,279			1,279	1,279
Cumulative Paid for Accident Years Displayed in Triangles	(1,262)			(1,262)	(1,262)
Unpaid for Accident Years not Displayed in Triangles	5			5	5
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	3			3	3
Discount	0			0	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	25			25	25
Reinsurance and Other Recoverables	0			0	0
Unpaid losses and loss adjustment expenses	25			25	25
Homeowners
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Paid for Accident Years Displayed in Triangles	(6,435)			(6,435)	(6,435)
Homeowners | Segment Reconciling Items
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	6,770			6,770	6,770
Cumulative Paid for Accident Years Displayed in Triangles	(6,435)			(6,435)	(6,435)
Unpaid for Accident Years not Displayed in Triangles	6			6	6
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	31			31	31
Discount	0			0	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	372			372	372
Reinsurance and Other Recoverables	3			3	3
Unpaid losses and loss adjustment expenses	375			375	375
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	24
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	208			208	208
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	(2)			(2)	(2)
Discount	(15)			(15)	(15)
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	191			191	191
Reinsurance and Other Recoverables	294			294	294
Unpaid losses and loss adjustment expenses	485			485	485
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	349			349	349
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	164			164	164
Discount	0			0	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	513			513	513
Reinsurance and Other Recoverables	(13)			(13)	(13)
Unpaid losses and loss adjustment expenses	500			500	500
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	64,238			64,238	64,238
Cumulative Paid for Accident Years Displayed in Triangles	(46,059)			(46,059)	(46,059)
Unpaid for Accident Years not Displayed in Triangles	5,147			5,147	5,147
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	938			938	938
Discount	(367)	(388)	$ (388)	(367)	(367)
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	23,897	22,986	20,352	23,897	23,897	$ 19,818
Reinsurance and Other Recoverables	5,725	5,275	4,232	5,725	5,725	3,957
Unpaid losses and loss adjustment expenses	29,622	28,261	24,584	$ 29,622	$ 29,622	$ 23,775
Policyholder Benefits and Claims Incurred, Ceded	$ 1,156	$ 826	$ 661