Claim and Claim Adjustment Expense Reserves (Specialty - Line of Business Composition) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 18,701	$ 17,885	$ 17,965	$ 18,070
Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	4,898	$ 4,676
Medical Professional Liability | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	1,520
Other Professional Liability and Management Liability | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	2,850
Surety | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	385
Warranty | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	34
Other | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 109