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Segment Reporting - Narrative (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2016
USD ($)
supplier
Dec. 31, 2015
USD ($)
supplier
customer
Dec. 31, 2014
USD ($)
supplier
customer
Segment Reporting [Abstract]      
Asset optimization revenue | $ $ 133.0 $ 154.1 $ 284.6
Cost of revenues | $ $ 133.6 $ 152.6 $ 282.3
Customer | Retail Natural Gas      
Segment Reporting Information [Line Items]      
Number of significant suppliers | supplier   1 1
Customer | Retail Natural Gas | Sales Revenue      
Segment Reporting Information [Line Items]      
Number of significant customers | customer   1 1
Customer | Retail Electricity      
Segment Reporting Information [Line Items]      
Number of significant suppliers | supplier 2 1 1