Employee Benefit Plans (Asset Allocation Targets Reflecting a Percentage of Total Assets by Asset Class) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total	$ 13,906	$ 12,807	$ 13,655
Pension Benefits | U.S. Equities
Defined Benefit Plan Disclosure [Line Items]
Total	475	518
Pension Benefits | International Equities
Defined Benefit Plan Disclosure [Line Items]
Total	753	640
Pension Benefits | Fixed Maturities
Defined Benefit Plan Disclosure [Line Items]
Total	8,844	8,038
Pension Benefits | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Total	91	77
Pension Benefits | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Total	1,458	1,238
Pension Benefits | Other
Defined Benefit Plan Disclosure [Line Items]
Total	2,285	2,296
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total	1,557	1,432	$ 1,615
Other Postretirement Benefits | U.S. Equities
Defined Benefit Plan Disclosure [Line Items]
Total	771	644
Other Postretirement Benefits | International Equities
Defined Benefit Plan Disclosure [Line Items]
Total	177	150
Other Postretirement Benefits | Fixed Maturities
Defined Benefit Plan Disclosure [Line Items]
Total	446	500
Other Postretirement Benefits | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Total	$ 163	$ 138
Minimum | Pension Benefits | U.S. Equities
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	2.00%
Minimum | Pension Benefits | International Equities
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	2.00%
Minimum | Pension Benefits | Fixed Maturities
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	54.00%
Minimum | Pension Benefits | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	0.00%
Minimum | Pension Benefits | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	2.00%
Minimum | Pension Benefits | Other
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	6.00%
Minimum | Other Postretirement Benefits | U.S. Equities
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	29.00%
Minimum | Other Postretirement Benefits | International Equities
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	2.00%
Minimum | Other Postretirement Benefits | Fixed Maturities
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	10.00%
Minimum | Other Postretirement Benefits | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	0.00%
Minimum | Other Postretirement Benefits | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	0.00%
Minimum | Other Postretirement Benefits | Other
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	0.00%
Maximum | Pension Benefits | U.S. Equities
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	8.00%
Maximum | Pension Benefits | International Equities
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	10.00%
Maximum | Pension Benefits | Fixed Maturities
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	66.00%
Maximum | Pension Benefits | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	12.00%
Maximum | Pension Benefits | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	16.00%
Maximum | Pension Benefits | Other
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	27.00%
Maximum | Other Postretirement Benefits | U.S. Equities
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	66.00%
Maximum | Other Postretirement Benefits | International Equities
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	21.00%
Maximum | Other Postretirement Benefits | Fixed Maturities
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	48.00%
Maximum | Other Postretirement Benefits | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	35.00%
Maximum | Other Postretirement Benefits | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	0.00%
Maximum | Other Postretirement Benefits | Other
Defined Benefit Plan Disclosure [Line Items]
Asset allocation targets	0.00%