Insurance Claim Reserves (details) - Claims and Claim Adjustment Expenses Reserves Summary - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	$ 42,540
Discount (Net of Reinsurance)	(1,030)
Subtotal: Net Claims and Allocated Claim Adjustment Expense Reserves	41,510
Reinsurance Recoverables on Unpaid Losses	8,140
Claims and Claim Adjustment Reserves	49,650	$ 47,949	$ 48,295
Discount on reinsurance recoverables for long-term disability and annuity claim payments included in discount (net of reinsurance)	72
Total reinsurance recoverables (on paid and unpaid losses)	8,309	8,287
Property casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	42,540
Discount (Net of Reinsurance)	(1,030)
Subtotal: Net Claims and Allocated Claim Adjustment Expense Reserves	41,510	39,948	39,823	$ 41,036
Reinsurance Recoverables on Unpaid Losses	8,123	7,981	8,449	8,788
Claims and Claim Adjustment Reserves	49,633	47,929	48,272	$ 49,824
Accident and health [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Reinsurance Recoverables on Unpaid Losses	17
Claims and Claim Adjustment Reserves	17	20
Business Insurance [Member] | General liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	6,852
Business Insurance [Member] | Commercial property [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	1,040
Business Insurance [Member] | Commercial multi-peril [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	3,487
Business Insurance [Member] | Automobile [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	2,406
Business Insurance [Member] | Workers' compensation [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	15,812
Bond & Specialty Insurance [Member] | General liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	1,995
Bond & Specialty Insurance [Member] | Fidelity and surety [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	460
Personal Insurance [Member] | Automobile [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	2,647
Personal Insurance [Member] | Homeowners (excluding Other) [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	1,125
Personal Insurance [Member] | International - Canada [Member] | Property casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	760
Reportable Segments [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Claims and Claim Adjustment Reserves	49,633	47,929	48,272
Reportable Segments [Member] | Property casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	36,584
Discount (Net of Reinsurance)	(1,025)
Subtotal: Net Claims and Allocated Claim Adjustment Expense Reserves	35,559
Reinsurance Recoverables on Unpaid Losses	2,837
Claims and Claim Adjustment Reserves	38,396
Reportable Segments [Member] | Business Insurance [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Claims and Claim Adjustment Reserves	40,352	39,555	39,983
Reportable Segments [Member] | Business Insurance [Member] | General liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	6,852
Discount (Net of Reinsurance)	(163)
Subtotal: Net Claims and Allocated Claim Adjustment Expense Reserves	6,689
Reinsurance Recoverables on Unpaid Losses	748
Claims and Claim Adjustment Reserves	7,437
Reportable Segments [Member] | Business Insurance [Member] | Commercial property [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	1,040
Subtotal: Net Claims and Allocated Claim Adjustment Expense Reserves	1,040
Reinsurance Recoverables on Unpaid Losses	358
Claims and Claim Adjustment Reserves	1,398
Reportable Segments [Member] | Business Insurance [Member] | Commercial multi-peril [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	3,487
Subtotal: Net Claims and Allocated Claim Adjustment Expense Reserves	3,487
Reinsurance Recoverables on Unpaid Losses	131
Claims and Claim Adjustment Reserves	3,618
Reportable Segments [Member] | Business Insurance [Member] | Automobile [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	2,406
Subtotal: Net Claims and Allocated Claim Adjustment Expense Reserves	2,406
Reinsurance Recoverables on Unpaid Losses	269
Claims and Claim Adjustment Reserves	2,675
Reportable Segments [Member] | Business Insurance [Member] | Workers' compensation [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	15,812
Discount (Net of Reinsurance)	(862)
Subtotal: Net Claims and Allocated Claim Adjustment Expense Reserves	14,950
Reinsurance Recoverables on Unpaid Losses	703
Claims and Claim Adjustment Reserves	15,653
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Claims and Claim Adjustment Reserves	3,421	3,323	3,615
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | General liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	1,995
Subtotal: Net Claims and Allocated Claim Adjustment Expense Reserves	1,995
Reinsurance Recoverables on Unpaid Losses	130
Claims and Claim Adjustment Reserves	2,125
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Fidelity and surety [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	460
Subtotal: Net Claims and Allocated Claim Adjustment Expense Reserves	460
Reinsurance Recoverables on Unpaid Losses	19
Claims and Claim Adjustment Reserves	479
Reportable Segments [Member] | Personal Insurance [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Claims and Claim Adjustment Reserves	5,860	$ 5,051	$ 4,674
Reportable Segments [Member] | Personal Insurance [Member] | Automobile [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	2,647
Subtotal: Net Claims and Allocated Claim Adjustment Expense Reserves	2,647
Reinsurance Recoverables on Unpaid Losses	450
Claims and Claim Adjustment Reserves	3,097
Reportable Segments [Member] | Personal Insurance [Member] | Homeowners (excluding Other) [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	1,125
Subtotal: Net Claims and Allocated Claim Adjustment Expense Reserves	1,125
Reinsurance Recoverables on Unpaid Losses	2
Claims and Claim Adjustment Reserves	1,127
Reportable Segments [Member] | Personal Insurance [Member] | International - Canada [Member] | Property casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	760
Subtotal: Net Claims and Allocated Claim Adjustment Expense Reserves	760
Reinsurance Recoverables on Unpaid Losses	27
Claims and Claim Adjustment Reserves	787
Reconciling Items [Member] | Other insurance contracts [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	3,858
Discount (Net of Reinsurance)	(5)
Subtotal: Net Claims and Allocated Claim Adjustment Expense Reserves	3,853
Reinsurance Recoverables on Unpaid Losses	2,130
Claims and Claim Adjustment Reserves	5,983
Reconciling Items [Member] | Unallocated loss adjustment expense reserve [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	2,042
Subtotal: Net Claims and Allocated Claim Adjustment Expense Reserves	2,042
Reinsurance Recoverables on Unpaid Losses	36
Claims and Claim Adjustment Reserves	2,078
Reconciling Items [Member] | Structured settements [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Reinsurance Recoverables on Unpaid Losses	3,106
Claims and Claim Adjustment Reserves	3,106
Reconciling Items [Member] | Other [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	56
Subtotal: Net Claims and Allocated Claim Adjustment Expense Reserves	56
Reinsurance Recoverables on Unpaid Losses	14
Claims and Claim Adjustment Reserves	$ 70