Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Significant Accounting Policies [Line Items]
Regulatory liabilities, noncurrent	$ 530	$ 542
Amortization Period, Transition Property (in years)	12 to 14 years
Capitalized Interest Costs, Including Allowance for Funds Used During Construction	$ 6	8	$ 10
Provision for doubtful accounts	1	1	3
Cash and cash equivalents	341	264	290	$ 207
Unamortized debt issuance expense	23	21
Removal Costs [Member]
Significant Accounting Policies [Line Items]
Regulatory liabilities, noncurrent	345	350
Other Income [Member]
Significant Accounting Policies [Line Items]
AFUDC equity	6	12	$ 14
Other Current Liabilities [Member]
Significant Accounting Policies [Line Items]
Regulatory liabilities, current	7	2
Customer deposits	17	12
Prepaid Expenses and Other Current Assets [Member]
Significant Accounting Policies [Line Items]
Restricted cash accounts	40	35
Variable Interest Entity, Primary Beneficiary [Member]
Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 340	$ 264