Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period			$ 11,455
(Gains) losses reclassified from net accumulated other comprehensive loss	$ 2	$ 2	3	$ 3
Accumulated other comprehensive income (loss) at end of period	11,650		11,650
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(58)	(51)	(58)	(51)
(Gains) losses reclassified from net accumulated other comprehensive loss	1	1	1	1
Net current period other comprehensive income (loss)	1	1	1	1
Accumulated other comprehensive income (loss) at end of period	(57)	(50)	(57)	(50)
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(66)	(58)	(67)	(59)
(Gains) losses reclassified from net accumulated other comprehensive loss	1	1	2	2
Net current period other comprehensive income (loss)	1	1	2	2
Accumulated other comprehensive income (loss) at end of period	(65)	(57)	(65)	(57)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(124)	(109)	(125)	(110)
(Gains) losses reclassified from net accumulated other comprehensive loss	2	2	3	3
Net current period other comprehensive income (loss)	2	2	3	3
Accumulated other comprehensive income (loss) at end of period	$ (122)	$ (107)	$ (122)	$ (107)