Segment Reporting - Narrative (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting [Abstract]
Asset optimization revenue	$ 284,600,000	$ 192,400,000	$ 248,600,000
Cost of revenues	$ 282,300,000	$ 192,100,000	$ 249,700,000
Retail Natural Gas | Customer
Segment Reporting Information [Line Items]
Number of significant customers		1	1
Number of significant suppliers	1
Retail Natural Gas | Customer | Sales Revenue
Segment Reporting Information [Line Items]
Number of significant customers	1	1	1
Retail Electricity | Customer
Segment Reporting Information [Line Items]
Number of significant suppliers	3	1	0