Regulatory Matters - Regulatory Liabilities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Regulatory Liabilities [Line Items]
Current liabilities	$ 39		$ 37
Noncurrent liabilities	262		243
Total regulatory liabilities	301		280
Cost of removal [Member]
Regulatory Liabilities [Line Items]
Regulatory liability amortization period years	P41Y	[1]
Total regulatory liabilities	233	[1]	219	[1]
Renewable energy program [Member]
Regulatory Liabilities [Line Items]
Regulatory liability amortization period years	P1Y
Total regulatory liabilities	32		24
Energy efficiency program [Member]
Regulatory Liabilities [Line Items]
Regulatory liability amortization period years	P1Y
Total regulatory liabilities	7		12
Deferred Income Taxes [Member]
Regulatory Liabilities [Line Items]
Regulatory liability amortization period years	P11Y
Total regulatory liabilities	8		9
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liability amortization period years	Various
Total regulatory liabilities	$ 21		$ 16

[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.