Asset Retirement Obligations (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2013	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Asset Retirement Obligation Disclosure [Abstract]
Proceeds from Decommissioning Trust Fund Assets				$ 50
Litigation Settlement, Amount	$ 1			79
Gain Contingency, Unrecorded Amount				2
Asset Retirement Obligation Rate Recovery related to Trojan Plant				4
Asset Retirement Obligation, Period Increase (Decrease)				$ 9
Asset Retirement Obligations, Significant Changes				6	17
Asset Retirement Obligation, Revision of Estimate		$ 3		$ 5	$ 30	$ 2
Asset Retirement Obligation, Accretion Expense				7	7	6
Asset Retirement Obligation, Liabilities Incurred				1	2	$ 15
Loss Contingency, Damages Awarded, Value			$ 81
Utility Plant [Domain]
Asset Retirement Obligation, Revision of Estimate				6	4
Asset Retirement Obligation, Accretion Expense				6
Asset Retirement Obligation, Liabilities Incurred				$ 3	$ 9