Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax		$ (3)	$ 0	$ (5)	$ 0
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax		0	0	1	0
Income Tax Expense (Benefit)		24	54	49	114
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		(9)	0	(14)	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[1]	0	1	1	2
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period				12,222
Net current period other comprehensive income (loss)		(8)	2	(11)	3
Accumulated other comprehensive income (loss) at end of period		12,366		12,366
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax		(3)	0	(5)	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		(10)	0	(17)	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[1]	0	0	0	0
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period		(66)	(58)	(60)	(58)
Net current period other comprehensive income (loss)		(9)	1	(15)	1
Accumulated other comprehensive income (loss) at end of period		(75)	(57)	(75)	(57)
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		1	0	3	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[1]	0	1	1	2
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period		(61)	(66)	(64)	(67)
Net current period other comprehensive income (loss)		1	1	4	2
Accumulated other comprehensive income (loss) at end of period		(60)	(65)	(60)	(65)
Reclassification from AOCI, Current Period, Tax		0	(1)	0	(1)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period		(127)	(124)	(124)	(125)
Accumulated other comprehensive income (loss) at end of period		(135)	(122)	(135)	(122)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Reclassification from AOCI, Current Period, Tax		0	0	0	0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income Tax Expense (Benefit)		0	0	0	0
Interest rate derivatives
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[2]	1	1	2	1
Interest rate derivatives | Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[2]	1	1	2	1
Interest rate derivatives | Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[2]	0	0	0	0
Interest rate derivatives | Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income Tax Expense (Benefit)		$ 0	$ 0	$ 0	$ 0

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

[2] ^(a)  Included in interest charges.