Claim and Claim Adjustment Expense Reserves (Reconciliation of Liability for Unpaid Claim and Claim Adjustment Expense) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 17,885	$ 17,965	$ 18,070	$ 18,249
Reinsurance receivables	3,835
Total gross liability for unpaid claim and claim adjustment expenses	21,720	21,984
Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	4,676	4,831
Reinsurance receivables	562
Commercial
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	7,849	7,998
Reinsurance receivables	807
International
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	1,628	1,524
Reinsurance receivables	248
Corporate & Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	175	$ 192
Reinsurance receivables	2,059
Life & Group
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	3,557
Reinsurance receivables	$ 159