Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020		Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	$ 13,239
Accumulated other comprehensive income (loss) at end of period	13,302
Other Comprehensive Income (Loss), Tax	(3)		$ (2)
Interest Rate Derivatives, Tax	(6)		25
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	(80)		(60)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(10)		(7)
Losses reclassified from net accumulated other comprehensive loss	0	[1]	0	[2]
Net current period other comprehensive income (loss)	(8)		(6)
Accumulated other comprehensive income (loss) at end of period	(88)		(66)
Gains and Losses on Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Other Comprehensive Income (Loss), Tax	(3)		(2)
Interest Rate Derivatives, Tax	0		0
Amortization of Net Actuarial Loss, Tax	0		0
Gains and Losses on Cash Flow Hedges | Interest Rate Swap
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Losses reclassified from net accumulated other comprehensive loss	2	[3]	1	[4]
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	(61)		(64)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0		2
Losses reclassified from net accumulated other comprehensive loss	1	[1]	1	[2]
Net current period other comprehensive income (loss)	1		3
Accumulated other comprehensive income (loss) at end of period	(60)		(61)
Defined Benefit Pension and Postretirement Items | Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Other Comprehensive Income (Loss), Tax	0		1
Interest Rate Derivatives, Tax	0		0
Amortization of Net Actuarial Loss, Tax	0		0
Defined Benefit Pension and Postretirement Items | Interest Rate Swap
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Losses reclassified from net accumulated other comprehensive loss	0	[3]	0	[4]
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(141)		(124)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(10)		(5)
Losses reclassified from net accumulated other comprehensive loss	1	[1]	1	[2]
Net current period other comprehensive income (loss)	(7)		(3)
Accumulated other comprehensive income (loss) at end of period	(148)		(127)
Total | Interest Rate Swap
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Losses reclassified from net accumulated other comprehensive loss	$ 2	[3]	$ 1	[4]

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

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

[3] Included in interest charges.

[4] Included in interest charges.