Acquisitions - Chanarambie/Viking (Details) $ in Millions		1 Months Ended	12 Months Ended
		Apr. 30, 2015 USD ($) MW	Dec. 31, 2015 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2014 USD ($)
Liabilities Assumed [Abstract]
Power Sales Agreements - Non-Current Liability			$ 138.1	$ 113.8
Goodwill			130.6	131.2	$ 2.9
Power Sales Agreements - Current Liability			23.3	$ 24.6
Chanarambie/Viking [Member]
Business Acquisition [Line Items]
Percentage of Voting Interests Acquired		100.00%
Name of Acquired Entity		Chanarambie/Viking
Payments to Acquire Business		$ 48.0
Generating Capacity (MW) | MW		97.5
Assets Acquired [Abstract]
Current Assets		$ 4.8
Property, Plant and Equipment		103.0
Other Non-Current Assets	[1]	1.0
Total Assets Acquired		108.8
Liabilities Assumed [Abstract]
Current Liabilities	[2]	6.7
Power Sales Agreements - Non-Current Liability		49.0
Non-Current Liabilities		5.1
Total Liabilities Assumed		60.8
Net Identifiable Assets Acquired		48.0
Goodwill		0.3
Tax Deductible Goodwill		0.0
Power Sales Agreements - Current Liability		$ 5.9
Acquisition-Related Costs			$ 0.2
Chanarambie/Viking [Member] | Chanarambie/Viking PPA (expires 2018) [Member]
Business Acquisition [Line Items]
Generating Capacity (MW) | MW		12
Chanarambie/Viking [Member] | Chanarambie/Viking PPA (expires 2023) [Member]
Business Acquisition [Line Items]
Generating Capacity (MW) | MW		85.5

[1] Included in Other Non-Current Assets was $0.3 million of goodwill. For tax purposes, the purchase price allocation resulted in no allocation to goodwill.

[2] Current Liabilities included $5.9 million related to the current portion of PSAs.