Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Significant Accounting Policies [Line Items]
Regulatory liabilities	$ 542	$ 537
Amortization Period, Transition Property (in years)	12 to 14 years
Capitalized Interest Costs, Including Allowance for Funds Used During Construction	$ 8	10	$ 10
Provision for doubtful accounts	1	3	2
Cash and cash equivalents ($264 and $290 related to VIEs, respectively)	264	290	207	$ 646
Customer deposits included in other current liabilities	12	24
Deferred Tax Asset, Reclassified To Deferred Tax Liability Noncurrent		2
Removal Costs [Member]
Significant Accounting Policies [Line Items]
Regulatory liabilities	350	353
Other Income [Member]
Significant Accounting Policies [Line Items]
AFUDC equity	12	14	$ 8
Prepaid Expenses and Other Current Assets [Member]
Significant Accounting Policies [Line Items]
Restricted cash accounts	35	47
Other Noncurrent Assets [Member]
Significant Accounting Policies [Line Items]
Unamortized Debt Issuance Expense	22	26
Variable Interest Entity, Primary Beneficiary [Member]
Significant Accounting Policies [Line Items]
Cash and cash equivalents ($264 and $290 related to VIEs, respectively)	$ 264	$ 290