Claim and Claim Adjustment Expense Reserves (Reconciliation of Liability for Unpaid Claim and Claim Adjustment Expense) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 17,965	$ 18,070	$ 18,249	$ 18,576
Reinsurance receivables	4,019
Claim and claim adjustment expenses	21,984	22,004
Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	4,831	5,007
Reinsurance receivables	634
Commercial
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	7,998	8,136
Reinsurance receivables	745
International
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	1,524	1,432
Reinsurance receivables	226
Corporate & Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	192	$ 205
Reinsurance receivables	2,233
Life and Group
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	3,420
Reinsurance receivables	$ 181