Retirement Benefits and Trusteed Assets (Pension Plan - Fair Value Measurements) (Details) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 4,273	$ 4,636	$ 4,012
Assets valued at NAV	1,181	1,201
Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	27	114
U.S. Large Cap Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	609	826
U.S. Small Cap and Mid Cap Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	124	234
Non-U.S. Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	586	822
Assets valued at NAV	240	280
Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,898	1,454
Assets valued at NAV	0	0
Hedge Funds and Similar Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	630	858
Assets valued at NAV	542	593
Private Equity and Other
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	399	328
Assets valued at NAV	399	328
Securities Lending
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	(30)	(66)
Securities Lending Collateral
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	30	66
Level 1
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,160	1,845
Level 1 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 1 | U.S. Large Cap Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	606	821
Level 1 | U.S. Small Cap and Mid Cap Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	123	229
Level 1 | Non-U.S. Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	337	529
Level 1 | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	6	1
Level 1 | Hedge Funds and Similar Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	88	265
Level 1 | Private Equity and Other
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 1 | Securities Lending
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	(22)	(53)
Level 1 | Securities Lending Collateral
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	22	53
Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,932	1,590
Level 2 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	27	114
Level 2 | U.S. Large Cap Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	3	5
Level 2 | U.S. Small Cap and Mid Cap Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1	5
Level 2 | Non-U.S. Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	9	13
Level 2 | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,892	1,453
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
Level 2 | Securities Lending
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	(8)	(13)
Level 2 | Securities Lending Collateral
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 8	$ 13