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Segment Reporting - Narrative (Details)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2015
USD ($)
customer
supplier
Sep. 30, 2014
USD ($)
customer
supplier
Sep. 30, 2015
USD ($)
customer
supplier
Sep. 30, 2014
USD ($)
customer
supplier
Segment Reporting [Abstract]        
Asset optimization revenue | $ $ 27.7 $ 45.9 $ 120.7 $ 225.4
Asset optimization cost of revenues | $ $ 28.2 $ 46.0 $ 119.4 $ 223.7
Customer | Sales Revenue        
Segment Reporting Information [Line Items]        
Number of significant customers 1 4 1 1
Customer | Retail Natural Gas | Sales Revenue        
Segment Reporting Information [Line Items]        
Number of significant customers 0 0 0 0
Customer | Retail Electricity | Sales Revenue        
Segment Reporting Information [Line Items]        
Number of significant customers 0 0 0 0
Supplier | Cost of Revenues        
Segment Reporting Information [Line Items]        
Number of significant suppliers | supplier 0 0 0 0
Supplier | Retail Natural Gas | Cost of Revenues        
Segment Reporting Information [Line Items]        
Number of significant suppliers | supplier 1 2 1 1
Supplier | Retail Electricity | Cost of Revenues        
Segment Reporting Information [Line Items]        
Number of significant suppliers | supplier 5 1 4 1