Claim and Claim Adjustment Expense Reserves (Reconciliation of Liability for Unpaid Claim and Claim Adjustment Expense) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 18,701	$ 17,885	$ 17,965	$ 18,070
Reinsurance receivables	4,005
Total gross liability for unpaid claim and claim adjustment expenses	22,706	21,720
Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Total Net Carried Claim and Claim Adjustment Expense Reserves	4,898	4,676
Reinsurance receivables	850
Commercial
Liability for Claims and Claims Adjustment Expense [Line Items]
Total Net Carried Claim and Claim Adjustment Expense Reserves	8,204	7,849
Reinsurance receivables	837
International
Liability for Claims and Claims Adjustment Expense [Line Items]
Total Net Carried Claim and Claim Adjustment Expense Reserves	1,822	1,628
Reinsurance receivables	269
Corporate & Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Total Net Carried Claim and Claim Adjustment Expense Reserves	162	$ 175
Reinsurance receivables	1,921
Life & Group
Liability for Claims and Claims Adjustment Expense [Line Items]
Total Net Carried Claim and Claim Adjustment Expense Reserves	3,615
Reinsurance receivables	$ 128