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Investments (Acquisitions) (Details)
$ in Millions
3 Months Ended
Feb. 01, 2017
USD ($)
MWh
project
Dec. 31, 2015
USD ($)
business
Jun. 30, 2016
USD ($)
Dec. 31, 2016
USD ($)
Investment Holdings [Line Items]        
After tax charge related to the buy-out of an earn-out provision     $ 13.0  
Minnesota solar garden development project | Subsequent event        
Investment Holdings [Line Items]        
Businesses acquired | project 4      
Aggregate purchase price $ 10.5      
Power generating capacity of acquired projects (in megawatts) | MWh 28      
Minnesota solar garden development project | Maximum        
Investment Holdings [Line Items]        
Purchase price       $ 41.9
Reimbursed project specific interconnection costs       $ 8.7
Series of Individually Immaterial Business Acquisitions        
Investment Holdings [Line Items]        
Businesses acquired | business   3    
Aggregate purchase price   $ 100.0    
Intangible Assets, Net (Including Goodwill)   $ 90.0