Claim and Claim Adjustment Expense Reserves (Reconciliation of Liability for Unpaid Claim and Claim Adjustment Expense) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 18,070	$ 18,249	$ 18,576	$ 18,927
Reinsurance receivables	3,934
Claim and claim adjustment expenses	22,004	22,343
Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	5,179	5,404
Reinsurance receivables	669
Commercial
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	7,964	8,305
Reinsurance receivables	621
International
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	1,432	1,201
Reinsurance receivables	204
Corporate & Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	205	$ 230
Reinsurance receivables	2,231
Life and Group
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	3,290
Reinsurance receivables	$ 209