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	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss adjustment expenses	$ 36,517	$ 33,029
Workers’ compensation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	18,990
Cumulative Paid for Accident Years Displayed in Triangles	(10,991)
Unpaid for Accident Years not Displayed in Triangles	2,446
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	345
Discount	(372)
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	10,418
Reinsurance and Other Recoverables	(2,093)
Unpaid losses and loss adjustment expenses	12,511
General liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	5,711
Cumulative Paid for Accident Years Displayed in Triangles	(2,828)
Unpaid for Accident Years not Displayed in Triangles	471
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	140
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	3,494
Reinsurance and Other Recoverables	(630)
Unpaid losses and loss adjustment expenses	4,124
Marine [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,226
Cumulative Paid for Accident Years Displayed in Triangles	(974)
Unpaid for Accident Years not Displayed in Triangles	19
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	8
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	279
Reinsurance and Other Recoverables	(135)
Unpaid losses and loss adjustment expenses	414
Package business
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	6,817
Cumulative Paid for Accident Years Displayed in Triangles	(5,215)
Unpaid for Accident Years not Displayed in Triangles	49
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	91
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,742
Reinsurance and Other Recoverables	(33)
Unpaid losses and loss adjustment expenses	1,775
Property | Property and Casualty, Commercial Insurance [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	2,939
Cumulative Paid for Accident Years Displayed in Triangles	(2,520)
Unpaid for Accident Years not Displayed in Triangles	29
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	13
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	461
Reinsurance and Other Recoverables	(162)
Unpaid losses and loss adjustment expenses	623
Automobile liability | Property and Casualty, Commercial Insurance [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	3,698
Cumulative Paid for Accident Years Displayed in Triangles	(2,739)
Unpaid for Accident Years not Displayed in Triangles	11
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	22
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	992
Reinsurance and Other Recoverables	(73)
Unpaid losses and loss adjustment expenses	1,065
Automobile liability | Property and Casualty, Personal Insurance [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	11,985
Cumulative Paid for Accident Years Displayed in Triangles	(10,518)
Unpaid for Accident Years not Displayed in Triangles	25
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	68
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,560
Reinsurance and Other Recoverables	(27)
Unpaid losses and loss adjustment expenses	1,587
Commercial automobile physical damage
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	206
Cumulative Paid for Accident Years Displayed in Triangles	(194)
Unpaid for Accident Years not Displayed in Triangles	3
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	0
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	15
Reinsurance and Other Recoverables	1
Unpaid losses and loss adjustment expenses	14
Professional liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,796
Cumulative Paid for Accident Years Displayed in Triangles	(863)
Unpaid for Accident Years not Displayed in Triangles	86
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	31
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,050
Reinsurance and Other Recoverables	(549)
Unpaid losses and loss adjustment expenses	1,599
Bond
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	637
Cumulative Paid for Accident Years Displayed in Triangles	(353)
Unpaid for Accident Years not Displayed in Triangles	29
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	24
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	337
Reinsurance and Other Recoverables	(13)
Unpaid losses and loss adjustment expenses	350
Assumed Reinsurance [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,005
Cumulative Paid for Accident Years Displayed in Triangles	(824)
Unpaid for Accident Years not Displayed in Triangles	4
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	5
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	190
Reinsurance and Other Recoverables	(25)
Unpaid losses and loss adjustment expenses	215
Personal automobile physical damage
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,531
Cumulative Paid for Accident Years Displayed in Triangles	(1,507)
Unpaid for Accident Years not Displayed in Triangles	6
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	3
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	33
Reinsurance and Other Recoverables	0
Unpaid losses and loss adjustment expenses	33
Homeowners
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	7,443
Cumulative Paid for Accident Years Displayed in Triangles	(6,958)
Unpaid for Accident Years not Displayed in Triangles	4
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	38
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	527
Reinsurance and Other Recoverables	(41)
Unpaid losses and loss adjustment expenses	568
Other ongoing business
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid for Accident Years not Displayed in Triangles	231
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	0
Discount	(16)
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	215
Reinsurance and Other Recoverables	(312)
Unpaid losses and loss adjustment expenses	527
Asbestos and Environmental
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid for Accident Years not Displayed in Triangles	1,147
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	0
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,147
Reinsurance and Other Recoverables	(1,219)
Unpaid losses and loss adjustment expenses	2,366
Other operations
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid for Accident Years not Displayed in Triangles	375
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	151
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	526
Reinsurance and Other Recoverables	36
Unpaid losses and loss adjustment expenses	490
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	63,984
Cumulative Paid for Accident Years Displayed in Triangles	(46,484)
Unpaid for Accident Years not Displayed in Triangles	4,935
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	939
Discount	(388)	$ (388)	$ (410)
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	22,986
Reinsurance and Other Recoverables	(5,275)
Unpaid losses and loss adjustment expenses	$ 28,261