Retirement Benefits and Trusteed Assets (Pension Plan - Fair Value Measurements) (Details) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 5,497	$ 4,993	$ 4,273
Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	92	99
Equity Securities, Domestic
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,260	1,042
Equity Securities, International
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	891	709
Fixed Income Securities, Governmental
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	554	569
Fixed Income Securities, Corporate
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,404	1,452
Hedge Funds and Similar Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	710	671
Private Equity and Other
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	586	451
Level 1
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,056	1,396
Level 1 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	92	99
Level 1 | Equity Securities, Domestic
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	167	172
Level 1 | Equity Securities, International
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	100	387
Level 1 | Fixed Income Securities, Governmental
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	459	569
Level 1 | Fixed Income Securities, Corporate
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 1 | Hedge Funds and Similar Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	238	169
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	1,560	1,452
Level 2 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 2 | Equity Securities, Domestic
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 2 | Equity Securities, International
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 2 | Fixed Income Securities, Governmental
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	95	0
Level 2 | Fixed Income Securities, Corporate
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,404	1,452
Level 2 | Hedge Funds and Similar Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	61	0
Level 2 | Private Equity and Other
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Other
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	2,881	2,145
Other | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Other | Equity Securities, Domestic
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,093	870
Other | Equity Securities, International
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	791	322
Other | Fixed Income Securities, Governmental
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Other | Fixed Income Securities, Corporate
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Other | Hedge Funds and Similar Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	411	502
Other | Private Equity and Other
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 586	$ 451