Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current income tax expense:
Federal	$ 70	$ 165	$ 106
State	19	18	21
Total current expense	89	183	127
Deferred income tax expense (benefit):
Federal	(98)	(34)	18
Total deferred expense (benefit)	(98)	(34)	18
Total income tax expense (benefit)	(9)	149	145
Income Tax Expense (Benefit) Reconciliation:
Income before income taxes	$ 424	$ 425	$ 406
Federal statutory income tax rate	35.00%	35.00%	35.00%
Expected federal income tax expense	$ 148	$ 149	$ 142
Increase (decrease) in tax expense resulting from:
State income tax expense, net of federal income tax	12	12	14
Federal income tax rate reduction	(158)	0	0
Other, net	(11)	(12)	(11)
Total	(157)	0	3
Total income tax expense (benefit)	$ (9)	$ 149	$ 145
Effective tax rate	(2.00%)	35.00%	36.00%
Deferred tax assets:
Benefits and compensation	$ 28	$ 47
Asset retirement obligations	7	12
Regulatory liabilities	198	17
Other	3	4
Total deferred tax assets	236	80
Deferred tax liabilities:
Property, plant and equipment	1,030	1,541
Regulatory assets	265	542
Total deferred tax liabilities	1,295	2,083
Net deferred tax liabilities	1,059	$ 2,003
Excess deferred income taxes [Member]
Deferred tax liabilities:
Net regulatory liability	829
Net regulatory liabilities gross-up	$ 180
Federal Income Tax Rate Prior To TCJA [Member]
Income Tax Expense (Benefit) Reconciliation:
Federal statutory income tax rate	35.00%
Federal Income Tax Rate After TCJA [Member]
Income Tax Expense (Benefit) Reconciliation:
Federal statutory income tax rate	21.00%