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Segment Reporting - Narrative (Details)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2015
USD ($)
supplier
customer
Jun. 30, 2014
USD ($)
supplier
customer
Jun. 30, 2015
USD ($)
supplier
customer
Jun. 30, 2014
USD ($)
supplier
customer
Segment Reporting [Abstract]        
Asset optimization revenue | $ $ 23.0 $ 46.7 $ 93.0 $ 179.6
Asset optimization cost of revenues | $ $ 23.1 $ 46.5 $ 91.1 $ 177.8
Customer | Sales Revenue        
Segment Reporting Information [Line Items]        
Number of significant customers | customer 1 4 1 1
Customer | Retail Natural Gas | Sales Revenue        
Segment Reporting Information [Line Items]        
Number of significant customers | customer 0 0 0 0
Customer | Retail Electricity | Sales Revenue        
Segment Reporting Information [Line Items]        
Number of significant customers | customer 0 0 0 0
Supplier | Cost of Revenues        
Segment Reporting Information [Line Items]        
Number of significant suppliers 0 0 0 0
Supplier | Retail Natural Gas | Cost of Revenues        
Segment Reporting Information [Line Items]        
Number of significant suppliers 3 3 1 2
Supplier | Retail Electricity | Cost of Revenues        
Segment Reporting Information [Line Items]        
Number of significant suppliers 2 1 3 1