Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current Income Tax Expense [Abstract]
Federal	$ 165	$ 106	$ 136
State	18	21	19
Total current expense	183	127	155
Deferred Income Tax Expense (Benefit) [Abstract]
Federal	(34)	18	(24)
Total deferred expense (benefit)	(34)	18	(24)
Total income tax expense	149	145	131
Income Tax Reconciliation [Abstract]
Income before income taxes	$ 425	$ 406	$ 383
Federal statutory income tax rate	35.00%	35.00%	35.00%
Expected federal income tax expense	$ 149	$ 142	$ 134
Increase (decrease) in tax expense resulting from:
State income tax expense, net of federal income tax	12	14	12
Other, net	(12)	(11)	(15)
Total	0	3	(3)
Total income tax expense	$ 149	$ 145	$ 131
Effective tax rate	35.00%	36.00%	34.00%
Tax benefits recognized			$ (6)
Deferred tax assets:
Benefits and compensation	$ 47	$ 69
Loss and credit carryforwards	0	6
AROs	12	13
Other	4	7
Total deferred tax assets	63	95
Deferred tax liabilities:
Property, Plant and Equipment	1,541	1,447
Regulatory assets/liabilities, net	525	680
Total deferred tax liabilities	2,066	2,127
Net deferred tax liabilities	$ 2,003	$ 2,032