Income Taxes Components of Net Deferred Income Tax Liability (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred income tax assets:
Regulatory liabilities	$ 362	$ 355
Employee benefits	184	98
Derivative contracts and unamortized contract values	79	76
State carryforwards	68	68
Loss contingencies	70	67
Asset retirement obligations	47	48
Other	92	86
Total deferred income tax assets	902	798
Deferred income tax liabilities:
Property, plant and equipment	(4,780)	(4,528)
Regulatory assets	(647)	(525)
Other	(56)	(38)
Total deferred income tax liabilities	(5,483)	(5,091)
Net deferred income tax liability	(4,581)	(4,293)
Reflected as:
Deferred income taxes - current assets	28	66
Deferred income taxes - noncurrent liabilities	$ (4,609)	$ (4,359)