Acquisitions - Chanarambie/Viking (Details) $ in Millions		1 Months Ended	3 Months Ended
		Apr. 30, 2015 USD ($) MW	Jun. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Liabilities Assumed [Abstract]
Power Purchase Agreements - Non-Current Liability				$ 119.8	$ 138.1
Goodwill				130.6	130.6
Power Purchase Agreements - Current Liability				$ 24.3	$ 23.3
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 Purchase 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 Purchase 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 PPAs.