Retirement Benefits and Trusteed Assets (OPEB - Target Allocations of Plan Assets) (Details) - Other postretirement benefit plan	Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Target allocation percentage of assets	100.00%
U.S. Large Cap Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Target allocation percentage of assets	11.00%
U.S. Small Cap and Mid Cap Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Target allocation percentage of assets	2.00%
Non-U.S. Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Target allocation percentage of assets	10.00%
Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Target allocation percentage of assets	52.00%
Hedge Funds and Similar Investments
Defined Benefit Plan Disclosure [Line Items]
Target allocation percentage of assets	11.00%
Private Equity and Other
Defined Benefit Plan Disclosure [Line Items]
Target allocation percentage of assets	14.00%