Regulatory Assets and Liabilities Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Remaining Recovery Period of Regulatory Assets, Price Risk Management	6 years
Remaining Recovery Period of Regulatory Asset, Debt Reacquisition Costs	6 years
Price risk management	$ 18	$ 10
Deferred income taxes	$ 29	42
Remaining refund period of regulatory liability, Trojan decommissioning activities	3 years
Current Regulatory Liabilities [Member]
Regulatory Assets [Line Items]
Asset retirement removal costs	$ 0	0
Proceeds from Legal Settlements	18	17
Asset retirement obligations	0	0
Other	33	38
Total regulatory liabilities	51	55
Noncurrent Regulatory Liabilities [Member]
Regulatory Assets [Line Items]
Asset retirement removal costs	887	837
Proceeds from Legal Settlements	0	15
Asset retirement obligations	49	45
Other	22	31
Total regulatory liabilities	958	928
Regulatory Assets, Current [Member]
Regulatory Assets [Line Items]
Price risk management	26	120
Pension and other postretirement plans	0	0
Deferred income taxes	0	0
Debt reacquisition costs	0	0
Other	10	9
Total regulatory assets	36	129
Noncurrent Regulatory Assets [Member]
Regulatory Assets [Line Items]
Price risk management	120	161
Pension and other postretirement plans	235	239
Deferred income taxes	86	86
Debt reacquisition costs	22	16
Other	35	22
Total regulatory assets	$ 498	$ 524