Retirement Benefits and Trusteed Assets (Pension Plan - Fair Value Measurements) (Details) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 4,993	$ 4,273	$ 4,636
Assets valued at NAV	2,145	1,181
Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	99	27
Equity Securities, Domestic
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,042	733
Assets valued at NAV	870	0
Equity Securities, International
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	709	586
Assets valued at NAV	322	240
Fixed Income Securities, Governmental
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	569	868
Assets valued at NAV	0	0
Fixed Income Securities, Corporate
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,452	1,030
Assets valued at NAV	0	0
Hedge Funds and Similar Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	671	630
Assets valued at NAV	502	542
Private Equity and Other
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	451	399
Assets valued at NAV	451	399
Level 1
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,396	1,160
Level 1 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	99	0
Level 1 | Equity Securities, Domestic
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	172	729
Level 1 | Equity Securities, International
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	387	337
Level 1 | Fixed Income Securities, Governmental
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	569	0
Level 1 | Fixed Income Securities, Corporate
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	6
Level 1 | Hedge Funds and Similar Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	169	88
Level 1 | Private Equity and Other
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,452	1,932
Level 2 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	27
Level 2 | Equity Securities, Domestic
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	4
Level 2 | Equity Securities, International
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	9
Level 2 | Fixed Income Securities, Governmental
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	868
Level 2 | Fixed Income Securities, Corporate
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,452	1,024
Level 2 | Hedge Funds and Similar Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 2 | Private Equity and Other
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 0	$ 0