Regulatory Assets (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Regulatory Assets [Line Items]
Regulatory assets, current	$ 253,000,000		$ 193,000,000
Regulatory assets, noncurrent	4,000,000,000		3,322,000,000
Total regulatory assets	4,253,000,000		3,515,000,000
Regulatory assets not earning a return on investment	2,600,000,000		2,200,000,000
Deferred income taxes [Member]
Regulatory Assets [Line Items]
Total regulatory assets	1,468,000,000	[1]	1,413,000,000	[1]
Weighted average remaining life	26 years
Employee benefit plans [Member]
Regulatory Assets [Line Items]
Total regulatory assets	747,000,000	[2]	589,000,000	[2]
Weighted average remaining life	9 years
Asset disposition costs [Member]
Regulatory Assets [Line Items]
Total regulatory assets	329,000,000	[3]	23,000,000	[3]
Weighted average remaining life	Various
Deferred net power costs [Member]
Regulatory Assets [Line Items]
Total regulatory assets	277,000,000		303,000,000
Weighted average remaining life	1 year
Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Total regulatory assets	239,000,000		193,000,000
Weighted average remaining life	10 years
Unrealized loss on regulated derivative contracts [Member]
Regulatory Assets [Line Items]
Total regulatory assets	223,000,000		182,000,000
Weighted average remaining life	5 years
Abandoned Projects [Member]
Regulatory Assets [Line Items]
Total regulatory assets	159,000,000		80,000,000
Weighted average remaining life	5 years
Unamortized contract values [Member]
Regulatory Assets [Line Items]
Total regulatory assets	123,000,000		146,000,000
Weighted average remaining life	8 years
Other [Member]
Regulatory Assets [Line Items]
Total regulatory assets	$ 688,000,000		$ 586,000,000
Weighted average remaining life	Various

[1] Amounts primarily represent income tax benefits related to state accelerated tax depreciation and certain property-related basis differences that were previously flowed through to customers and will be included in regulated rates when the temporary differences reverse.

[2] Represents amounts not yet recognized as a component of net periodic benefit cost that are expected to be included in regulated rates when recognized.

[3] Includes amounts established as a result of the Utah Mine Disposition for the net property, plant and equipment not considered probable of disallowance and for the portion of losses associated with the assets held for sale, UMWA 1974 Pension Trust withdrawal and closure costs incurred to date considered probable of recovery.