Insurance Claim Reserves (Details) - Claims and Claim Adjustment Expenses Reserves Summary - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	$ 43,559
Discount (Net of Reinsurance)	(1,088)
Subtotal: Net Claims and Claim Adjustment Expense Reserves	42,471
Reinsurance Recoverables on Unpaid Losses	8,197
Claims and Claim Adjustment Expense Reserves	50,668	$ 49,650
Discount on reinsurance recoverables for long-term disability and annuity claim payments included in discount (net of reinsurance)	70
Total reinsurance recoverables	8,370	8,309
Property-casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	43,559
Discount (Net of Reinsurance)	(1,088)
Subtotal: Net Claims and Claim Adjustment Expense Reserves	42,471	41,510	$ 39,948	$ 39,823
Reinsurance Recoverables on Unpaid Losses	8,182	8,123	7,981	8,449
Claims and Claim Adjustment Expense Reserves	50,653	49,633	$ 47,929	$ 48,272
Accident and health [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Reinsurance Recoverables on Unpaid Losses	15
Claims and Claim Adjustment Expense Reserves	15	$ 17
Business Insurance [Member] | General liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	7,159
Business Insurance [Member] | Commercial property [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	973
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	3,535
Business Insurance [Member] | Automobile [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	2,861
Business Insurance [Member] | Workers' compensation [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	16,039
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	1,833
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	419
Personal Insurance [Member] | Automobile [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	2,776
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	1,376
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	710
Reportable segments [Member] | Property-casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	37,681
Discount (Net of Reinsurance)	(1,083)
Subtotal: Net Claims and Claim Adjustment Expense Reserves	36,598
Reinsurance Recoverables on Unpaid Losses	3,056
Claims and Claim Adjustment Expense Reserves	39,654
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	7,159
Discount (Net of Reinsurance)	(174)
Subtotal: Net Claims and Claim Adjustment Expense Reserves	6,985
Reinsurance Recoverables on Unpaid Losses	854
Claims and Claim Adjustment Expense Reserves	7,839
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	973
Subtotal: Net Claims and Claim Adjustment Expense Reserves	973
Reinsurance Recoverables on Unpaid Losses	429
Claims and Claim Adjustment Expense Reserves	1,402
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	3,535
Subtotal: Net Claims and Claim Adjustment Expense Reserves	3,535
Reinsurance Recoverables on Unpaid Losses	181
Claims and Claim Adjustment Expense Reserves	3,716
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	2,861
Subtotal: Net Claims and Claim Adjustment Expense Reserves	2,861
Reinsurance Recoverables on Unpaid Losses	226
Claims and Claim Adjustment Expense Reserves	3,087
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	16,039
Discount (Net of Reinsurance)	(909)
Subtotal: Net Claims and Claim Adjustment Expense Reserves	15,130
Reinsurance Recoverables on Unpaid Losses	698
Claims and Claim Adjustment Expense Reserves	15,828
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	1,833
Subtotal: Net Claims and Claim Adjustment Expense Reserves	1,833
Reinsurance Recoverables on Unpaid Losses	152
Claims and Claim Adjustment Expense Reserves	1,985
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	419
Subtotal: Net Claims and Claim Adjustment Expense Reserves	419
Reinsurance Recoverables on Unpaid Losses	7
Claims and Claim Adjustment Expense Reserves	426
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	2,776
Subtotal: Net Claims and Claim Adjustment Expense Reserves	2,776
Reinsurance Recoverables on Unpaid Losses	480
Claims and Claim Adjustment Expense Reserves	3,256
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	1,376
Subtotal: Net Claims and Claim Adjustment Expense Reserves	1,376
Reinsurance Recoverables on Unpaid Losses	3
Claims and Claim Adjustment Expense Reserves	1,379
Reportable segments [Member] | Personal Insurance [Member] | International - Canada [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	710
Subtotal: Net Claims and Claim Adjustment Expense Reserves	710
Reinsurance Recoverables on Unpaid Losses	26
Claims and Claim Adjustment Expense Reserves	736
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	3,762
Discount (Net of Reinsurance)	(5)
Subtotal: Net Claims and Claim Adjustment Expense Reserves	3,757
Reinsurance Recoverables on Unpaid Losses	2,116
Claims and Claim Adjustment Expense Reserves	5,873
Reconciling Items [Member] | Unallocated loss adjustment expense reserves [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	2,053
Subtotal: Net Claims and Claim Adjustment Expense Reserves	2,053
Reinsurance Recoverables on Unpaid Losses	37
Claims and Claim Adjustment Expense Reserves	2,090
Reconciling Items [Member] | Structured settlements [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Reinsurance Recoverables on Unpaid Losses	2,990
Claims and Claim Adjustment Expense Reserves	2,990
Reconciling Items [Member] | Other [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	63
Subtotal: Net Claims and Claim Adjustment Expense Reserves	63
Reinsurance Recoverables on Unpaid Losses	(17)
Claims and Claim Adjustment Expense Reserves	$ 46