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
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss adjustment expenses	$ 32,287	$ 28,317
Workers’ compensation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	18,351
Cumulative Paid for Accident Years Displayed in Triangles	(10,945)
Unpaid for Accident Years not Displayed in Triangles	2,242
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	346
Discount	(394)
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	9,600
Reinsurance and Other Recoverables	2,166
Unpaid losses and loss adjustment expenses	11,766
Liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	3,473
Cumulative Paid for Accident Years Displayed in Triangles	(1,894)
Unpaid for Accident Years not Displayed in Triangles	500
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	88
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	2,167
Reinsurance and Other Recoverables	239
Unpaid losses and loss adjustment expenses	2,406
Package business
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	6,553
Cumulative Paid for Accident Years Displayed in Triangles	(5,198)
Unpaid for Accident Years not Displayed in Triangles	46
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	99
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,500
Reinsurance and Other Recoverables	45
Unpaid losses and loss adjustment expenses	1,545
Commercial property
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	3,263
Cumulative Paid for Accident Years Displayed in Triangles	(2,898)
Unpaid for Accident Years not Displayed in Triangles	15
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	10
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	390
Reinsurance and Other Recoverables	53
Unpaid losses and loss adjustment expenses	443
Commercial automobile liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	3,446
Cumulative Paid for Accident Years Displayed in Triangles	(2,559)
Unpaid for Accident Years not Displayed in Triangles	17
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	23
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	927
Reinsurance and Other Recoverables	41
Unpaid losses and loss adjustment expenses	968
Commercial automobile physical damage
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	238
Cumulative Paid for Accident Years Displayed in Triangles	(227)
Unpaid for Accident Years not Displayed in Triangles	2
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	0
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	13
Reinsurance and Other Recoverables	0
Unpaid losses and loss adjustment expenses	13
Professional liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,647
Cumulative Paid for Accident Years Displayed in Triangles	(1,146)
Unpaid for Accident Years not Displayed in Triangles	43
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	17
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	561
Reinsurance and Other Recoverables	282
Unpaid losses and loss adjustment expenses	843
Bond
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	598
Cumulative Paid for Accident Years Displayed in Triangles	(333)
Unpaid for Accident Years not Displayed in Triangles	4
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	17
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	286
Reinsurance and Other Recoverables	15
Unpaid losses and loss adjustment expenses	301
Personal automobile liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	12,363
Cumulative Paid for Accident Years Displayed in Triangles	(10,749)
Unpaid for Accident Years not Displayed in Triangles	19
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	74
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,707
Reinsurance and Other Recoverables	21
Unpaid losses and loss adjustment expenses	1,728
Personal automobile physical damage
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,884
Cumulative Paid for Accident Years Displayed in Triangles	(1,856)
Unpaid for Accident Years not Displayed in Triangles	1
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	3
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	32
Reinsurance and Other Recoverables	0
Unpaid losses and loss adjustment expenses	32
Homeowners
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	7,588
Cumulative Paid for Accident Years Displayed in Triangles	(7,161)
Unpaid for Accident Years not Displayed in Triangles	4
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	40
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	471
Reinsurance and Other Recoverables	50
Unpaid losses and loss adjustment expenses	521
Other ongoing business
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid for Accident Years not Displayed in Triangles	204
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	0
Discount	(16)
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	188
Reinsurance and Other Recoverables	308
Unpaid losses and loss adjustment expenses	496
Asbestos and Environmental
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid for Accident Years not Displayed in Triangles	1,452	1,700
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	0
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	1,452
Reinsurance and Other Recoverables	765
Unpaid losses and loss adjustment expenses	2,217	1,700
Other operations
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid for Accident Years not Displayed in Triangles	402
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	122
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	524
Reinsurance and Other Recoverables	(28)
Unpaid losses and loss adjustment expenses	496
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	59,404
Cumulative Paid for Accident Years Displayed in Triangles	(44,966)
Unpaid for Accident Years not Displayed in Triangles	4,951
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	839
Discount	(410)	(483)	$ (523)
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	19,818	19,057	18,943	$ 18,765
Reinsurance and Other Recoverables	3,957	3,488	3,625	3,814
Unpaid losses and loss adjustment expenses	$ 23,775	22,545	$ 22,568	$ 22,579
Maximum | Property and Casualty Insurance Products
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Period for long-tail lines included in loss triangles	10 years
Period for short-tail lines included in loss triangles	3 years
Operating Segments | Commercial Lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss adjustment expenses	$ 18,893	$ 17,950