Claim and Claim Adjustment Expense Reserves (Gross and net carried claim and claim adjustment expense reserves) (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]
Gross Case Reserves	$ 11,395	$ 11,670
Gross IBNR Reserves	10,589	10,334
Total Gross Carried Claim and Claim Adjustment Expense Reserves	21,984	22,004	$ 22,343	$ 22,663
Net Case Reserves	9,523	9,645
Net IBNR Reserves	8,442	8,425
Total Net Carried Claim and Claim Adjustment Expense Reserves	17,965	18,070	$ 18,249	$ 18,576
Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	1,623	1,742
Gross IBNR Reserves	3,842	3,927
Total Gross Carried Claim and Claim Adjustment Expense Reserves	5,465	5,669
Net Case Reserves	1,483	1,600
Net IBNR Reserves	3,348	3,407
Total Net Carried Claim and Claim Adjustment Expense Reserves	4,831	5,007
Commercial
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	4,181	4,427
Gross IBNR Reserves	4,562	4,337
Total Gross Carried Claim and Claim Adjustment Expense Reserves	8,743	8,764
Net Case Reserves	3,831	4,103
Net IBNR Reserves	4,167	4,033
Total Net Carried Claim and Claim Adjustment Expense Reserves	7,998	8,136
International
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	867	744
Gross IBNR Reserves	883	892
Total Gross Carried Claim and Claim Adjustment Expense Reserves	1,750	1,636
Net Case Reserves	749	640
Net IBNR Reserves	775	792
Total Net Carried Claim and Claim Adjustment Expense Reserves	1,524	1,432
Life & Group
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	3,516	3,386
Gross IBNR Reserves	85	113
Total Gross Carried Claim and Claim Adjustment Expense Reserves	3,601	3,499
Net Case Reserves	3,364	3,208
Net IBNR Reserves	56	82
Total Net Carried Claim and Claim Adjustment Expense Reserves	3,420	3,290
Corporate & Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	1,208	1,371
Gross IBNR Reserves	1,217	1,065
Total Gross Carried Claim and Claim Adjustment Expense Reserves	2,425	2,436
Net Case Reserves	96	94
Net IBNR Reserves	96	111
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 192	$ 205