Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Current Income Tax Expense [Abstract]
Federal	$ 106	$ 136	$ 201
State	21	19	20
Total current expense	127	155	221
Deferred Income Tax Expense (Benefit) [Abstract]
Federal	18	(24)	(75)
Total deferred expense (benefit)	18	(24)	(75)
Total income tax expense	145	131	146
Income Tax Reconciliation [Abstract]
Income before income taxes	$ 406	$ 383	$ 415
Federal statutory income tax rate	35.00%	35.00%	35.00%
Expected federal income tax expense	$ 142	$ 134	$ 145
Increase (decrease) in tax expense resulting from:
State income tax expense, net of federal income tax	14	12	13
Decrease in settled and uncertain tax positions	0	0	(5)
Other, net	(11)	(15)	(7)
Total	3	(3)	1
Total income tax expense	$ 145	$ 131	$ 146
Effective tax rate	35.70%	34.20%	35.20%
Tax benefits recognized		$ (6)	$ (5)
Deferred tax assets:
Benefits and compensation	$ 69	92
Loss and credit carryforwards	6	0
Asset retirement obligations	13	12
Other	7	7
Total deferred tax assets	95	111
Deferred tax liabilities:
Deferred Tax Liabilities, Property, Plant and Equipment	1,447	1,284
Regulatory assets/liabilities, net	680	826
Total deferred tax liabilities	2,127	2,110
Net deferred tax liabilities	$ 2,032	$ 1,999