Retirement Benefits and Trusteed Assets (Pension Plan - Fair Value Measurements) (Details) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 4,636	$ 4,012	$ 3,832
Assets valued at NAV	1,201	1,112
Short-term investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	114	22
U.S. Large Capitalization (Cap) Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	826	880
U.S. Small Cap and Mid Cap Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	234	232
Non-U.S. Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	822	766
Assets valued at NAV	280	269
Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,454	1,090
Assets valued at NAV	0	52
Hedge Funds and Similar Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	858	809
Assets valued at NAV	593	578
Private Equity and Other
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	328	213
Assets valued at NAV	328	213
Securities lending
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	(66)	(78)
Securities lending collateral
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	66	78
Level 1
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,845	1,815
Level 1 | Short-term investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Level 1 | U.S. Large Capitalization (Cap) Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	821	875
Level 1 | U.S. Small Cap and Mid Cap Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	229	229
Level 1 | Non-U.S. Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	529	479
Level 1 | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1	1
Level 1 | Hedge Funds and Similar Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	265	231
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	(53)	(53)
Level 1 | Securities lending collateral
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	53	53
Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,590	1,085
Level 2 | Short-term investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	114	22
Level 2 | U.S. Large Capitalization (Cap) Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	5	5
Level 2 | U.S. Small Cap and Mid Cap Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	5	3
Level 2 | Non-U.S. Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	13	18
Level 2 | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,453	1,037
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	(13)	(25)
Level 2 | Securities lending collateral
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 13	$ 25