Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Other Investments	$ 0.0	$ 120.0
Business days after the invoice date	16
Days after the due date	45
Wholesale accounts receivable written off	$ 0.0	0.0	$ 0.0
Margin deposit	33.0	11.0
Letters of Credit Outstanding, Amount	63.0	30.0
Expenses related to Cascade Crossing	$ 0.0	$ 0.0	$ 52.0
Public Utilities, Allowance for Funds Used During Construction, Rate	7.30%	7.40%	7.50%
Public Utilities, Allowance for Funds Used During Construction, Capitalized Interest	$ 13.0	$ 22.0	$ 7.0
Allowance for equity funds used during construction	21.0	37.0	$ 13.0
Construction in Progress, Gross	424.0
Public Utilities, Allowance for Funds Used During Construction, Additions	41.0
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	545.0	$ 417.0
Malpractice Loss Contingency, Letters of Credit and Surety Bonds	$ 145.6
Depreciation expense rate	3.60%	3.60%	3.70%
Asset Retirement Removal Costs Included in Depreciation Expense	$ 32.0	$ 57.0	$ 55.0
Finite-Lived Intangible Assets, Accumulated Amortization	227.0	191.0
Amortization of Intangible Assets	38.0	$ 25.0	$ 22.0
Future Amortization Expense, Year One	43.0
Future Amortization Expense, Year Two	40.0
Future Amortization Expense, Year Three	39.0
Future Amortization Expense, Year Four	33.0
Future Amortization Expense, Year Five	23.0
Power Cost Deadband - Lower Threshold	15.0
Power Cost Deadband - Upper Threshold	$ 30.0
Public Utilities, Approved Return on Equity, Percentage	9.68%	9.75%	10.00%
Franchise taxes	$ 43.0	$ 42.0	$ 41.0
Deferred Tax Assets, Regulatory Assets and Liabilities	42.0	48.0
Type of Arrangement [Member]
Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	514.0
Noncurrent Regulatory Assets [Member]
Property, Plant and Equipment [Line Items]
Deferred Tax Assets, Regulatory Assets and Liabilities	86.0	86.0
Noncurrent Regulatory Liabilities [Member]
Property, Plant and Equipment [Line Items]
Asset Retirement Obligations, Noncurrent	45.0	$ 39.0
Maximum [Member]
Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	655.0
Minimum [Member]
Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	$ 620.0