Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	$ 6,996
Income tax expense (benefit)	45		$ 80		$ 113
Accumulated other comprehensive loss at end of period	7,592		6,996
Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(24)		(26)
Amortization of net actuarial income (loss)	0		0
Net current period other comprehensive income (loss)	1		2
Accumulated other comprehensive loss at end of period	(23)		(24)		(26)
Gains and Losses on Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Income tax expense (benefit)	0		0
Reclassification from AOCI, Current Period, Tax	0		0
Gains and Losses on Cash Flow Hedges | Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amortization of net actuarial income (loss)	(1)	[1]	(2)	[2]
Defined Benefit Pension and Postretirement Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(3)		0
Amortization of net actuarial income (loss)	(2)		3	[3]
Net current period other comprehensive income (loss)	2		(3)
Accumulated other comprehensive loss at end of period	(1)		(3)		0
Defined Benefit Pension and Postretirement Items | Reclassification out of Accumulated Other Comprehensive Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Income tax expense (benefit)	0		0
Reclassification from AOCI, Current Period, Tax	1		(1)
Defined Benefit Pension and Postretirement Items | Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amortization of net actuarial income (loss)	0		0
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(27)		(26)
Amortization of net actuarial income (loss)	(2)		3
Net current period other comprehensive income (loss)	3		(1)
Accumulated other comprehensive loss at end of period	(24)		(27)		$ (26)
Total | Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amortization of net actuarial income (loss)	$ (1)		$ (2)

[1] Included in interest charges.

[2] Included in interest charges.

[3] Included in the computation of net periodic pension and postretirement benefit costs. See Note 9 for further information.