Segment Reporting - Financial Data (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total Revenues	[1],[2]	$ 151,436	$ 109,381	$ 347,743	$ 219,927
Retail cost of revenues	[1],[2],[3]	114,637	56,963	260,398	125,763
Less:
Net asset optimization (expense)/revenues	[1],[2],[4]	(168)	(677)	(361)	(150)
Losses on non-trading derivatives		(9,677)	13,245	(31,473)	3,496
Current period settlements on non-trading derivatives		3,996	1,024	11,351	12,296
Retail Gross Margin		43,149	38,820	107,749	78,382
Total Assets		408,713		408,713		$ 376,168
Goodwill		80,947		80,947		79,147
Non-trading
Less:
Losses on non-trading derivatives		(10,202)	13,322	(31,578)	3,702
Current period settlements on non-trading derivatives		4,020	953	11,535	12,230
Operating Segments | Retail Electricity
Segment Reporting Information [Line Items]
Total Revenues		131,908	87,395	265,602	149,328
Retail cost of revenues		102,079	49,717	210,923	96,017
Less:
Net asset optimization (expense)/revenues		0	0	0	0
Retail Gross Margin		34,863	27,631	71,634	43,037
Total Assets		838,978		838,978		577,695
Goodwill		78,417		78,417		76,617
Operating Segments | Retail Electricity | Non-trading
Less:
Losses on non-trading derivatives		(9,333)	10,319	(28,960)	929
Current period settlements on non-trading derivatives		4,299	(272)	12,005	9,345
Operating Segments | Retail Natural Gas
Segment Reporting Information [Line Items]
Total Revenues		19,528	21,986	82,141	70,599
Retail cost of revenues		12,558	7,246	49,475	29,746
Less:
Net asset optimization (expense)/revenues		(168)	(677)	(361)	(150)
Retail Gross Margin		8,286	11,189	36,115	35,345
Total Assets		302,637		302,637		242,739
Goodwill		2,530		2,530		2,530
Operating Segments | Retail Natural Gas | Non-trading
Less:
Losses on non-trading derivatives		(869)	3,003	(2,618)	2,773
Current period settlements on non-trading derivatives		(279)	1,225	(470)	2,885
Corporate and Other
Segment Reporting Information [Line Items]
Total Revenues		0	0	0	0
Retail cost of revenues		0	0	0	0
Less:
Net asset optimization (expense)/revenues		0	0	0	0
Retail Gross Margin		0	0	0	0
Total Assets		247,693		247,693		169,404
Goodwill		0	0	0	0	0
Corporate and Other | Non-trading
Less:
Losses on non-trading derivatives		0	0	0	0
Current period settlements on non-trading derivatives		0	0	0	0
Eliminations
Segment Reporting Information [Line Items]
Total Revenues		0	0	0	0
Retail cost of revenues		0	0	0	0
Less:
Net asset optimization (expense)/revenues		0	0	0	0
Retail Gross Margin		0	0	0	0
Total Assets		(980,595)		(980,595)		(613,670)
Goodwill		0		0		$ 0
Eliminations | Non-trading
Less:
Losses on non-trading derivatives		0	0	0	0
Current period settlements on non-trading derivatives		$ 0	$ 0	$ 0	$ 0

[1] Financial information has been recast to include results attributable to the acquisition of Perigee Energy, LLC by an affiliate on February 3, 2017. See Notes 2 and 3 "Basis of Presentation and Summary of Significant Accounting Policies" and "Acquisitions" for further discussion.

[2] Financial information has been recast to include results attributable to the acquisition of the Major Energy Companies by an affiliate on April 15, 2016. See Note 2 and 3 "Basis of Presentation and Summary of Significant Accounting Policies" and "Acquisitions" for further discussion.

[3] Retail cost of revenues includes retail cost of revenues—affiliates of $0 and less than $100 for the three months ended June 30, 2017 and 2016, respectively, and $0 and less than $100 for the six months ended June 30, 2017 and 2016, respectively.

[4] Net asset optimization revenues (expenses) includes asset optimization revenues—affiliates of $0 and $41 for the three months ended June 30, 2017 and 2016, respectively, and asset optimization revenues—affiliates cost of revenues of $0 and $376 for the three months ended June 30, 2017 and 2016, respectively, and asset optimization revenues—affiliates of $0 and $154 for the six months ended June 30, 2017 and 2016, respectively, and asset optimization revenue—affiliates cost of revenues of $0 and $1,633 for the six months ended June 30, 2017 and 2016, respectively.