Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019		Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	$ 12,222
Accumulated other comprehensive income (loss) at end of period	13,239		$ 12,222
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	(60)		(58)
Other comprehensive loss before reclassifications	(23)		(5)
Amortization of net actuarial loss	0		0
Net current period other comprehensive income (loss)	(20)		(2)
Accumulated other comprehensive income (loss) at end of period	(80)		(60)
Gains and Losses on Cash Flow Hedges | Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amortization of net actuarial loss	(3)	[1]	(3)	[2]
Defined Benefit Pension and Postretirement Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(64)		(67)
Other comprehensive loss before reclassifications	0		(6)
Amortization of net actuarial loss	(3)	[3]	(9)	[4]
Net current period other comprehensive income (loss)	3		3
Accumulated other comprehensive income (loss) at end of period	(61)		(64)
Defined Benefit Pension and Postretirement Items | Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amortization of net actuarial loss	0		0
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(124)		(125)
Other comprehensive loss before reclassifications	(23)		(11)
Amortization of net actuarial loss	(3)		(9)
Net current period other comprehensive income (loss)	(17)		1
Accumulated other comprehensive income (loss) at end of period	(141)		(124)
Total | Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amortization of net actuarial loss	$ (3)		$ (3)

[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 11 for further information.

[4] Included in the computation of net periodic pension and postretirement benefit costs.