Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss) before Reclassifications, Tax	$ (2)	$ 0	$ 0
Income tax expense (benefit)	181	542	581
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, Tax	(2)	(2)	(5)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	11,455
Other comprehensive loss before reclassifications	(11)	(3)
Amortization of net actuarial loss	9	7
Net current period other comprehensive income (loss)	1	7
Adoption of ASU No. 2018-02		0
Accumulated other comprehensive income (loss) at end of period	12,222	11,455
Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(58)	(51)
Other comprehensive loss before reclassifications	(5)	0
Amortization of net actuarial loss	0	0
Net current period other comprehensive income (loss)	(2)	3
Adoption of ASU No. 2018-02		(10)
Accumulated other comprehensive income (loss) at end of period	(60)	(58)	(51)
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, Current Period, Tax	3	5
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(67)	(59)
Other comprehensive loss before reclassifications	(6)	(3)
Amortization of net actuarial loss	9	7
Net current period other comprehensive income (loss)	3	4
Adoption of ASU No. 2018-02		(12)
Accumulated other comprehensive income (loss) at end of period	(64)	(67)	(59)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(125)	(110)
Adoption of ASU No. 2018-02		(22)
Accumulated other comprehensive income (loss) at end of period	(124)	(125)	$ (110)
Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amortization of net actuarial loss	3	3
Interest Rate Swap | Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amortization of net actuarial loss	3	3
Interest Rate Swap | Defined Benefit Pension and Postretirement Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amortization of net actuarial loss	0	0
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income tax expense (benefit)	0	0
Reclassification from AOCI, Current Period, Tax	0	0
Reclassification out of Accumulated Other Comprehensive Income | Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income tax expense (benefit)	$ 1	$ 2