Regulatory Liabilities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	$ 163		$ 167
Regulatory liabilities, noncurrent	2,669		2,498
Total regulatory liabilities	2,832		2,665
Cost of removal [Member]
Regulatory Liabilities [Line Items]
Total regulatory liabilities	2,215	[1]	2,009	[1]
Weighted average remaining life	28 years
Asset retirement obligations [Member]
Regulatory Liabilities [Line Items]
Total regulatory liabilities	169		151
Weighted average remaining life	23 years
Levelized depreciation [Member]
Regulatory Liabilities [Line Items]
Total regulatory liabilities	169		144
Weighted average remaining life	27 years
Employee benefit plans [Member]
Regulatory Liabilities [Line Items]
Total regulatory liabilities	20	[2]	74	[2]
Weighted average remaining life	12 years
Other [Member]
Regulatory Liabilities [Line Items]
Total regulatory liabilities	$ 259		$ 287
Weighted average remaining life	Various

[1] Amounts represent estimated costs, as accrued through depreciation rates and exclusive of ARO liabilities, of removing regulated property, plant and equipment in accordance with accepted regulatory practices. Amounts are deducted from rate base or otherwise accrue a carrying cost.

[2] Represents amounts not yet recognized as a component of net periodic benefit cost that are to be returned to customers in future periods when recognized.