Segment Reporting - Financial Data for Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total Revenues	$ 166,681	$ 242,706
Retail cost of revenues	118,823	195,255
Less:
Net asset optimization (expenses) revenues	321	2,552
Net, gain (loss) on non-trading derivative instruments	(24,587)	(19,541)
Current period settlements on non-trading derivatives	16,608	8,025
Retail Gross Margin	55,461	56,577
Total Assets	389,632		$ 422,968
Goodwill	120,343		120,343
Non-trading
Less:
Net, gain (loss) on non-trading derivative instruments	(24,533)	(19,803)
Current period settlements on non-trading derivatives	16,609	8,125
Operating Segments | Retail Electricity
Segment Reporting Information [Line Items]
Total Revenues	121,768	182,092
Retail cost of revenues	100,383	165,888
Less:
Net asset optimization (expenses) revenues	0	0
Retail Gross Margin	30,806	29,973
Total Assets	2,622,259		2,524,884
Goodwill	117,813		117,813
Operating Segments | Retail Electricity | Non-trading
Less:
Net, gain (loss) on non-trading derivative instruments	(24,386)	(21,942)
Current period settlements on non-trading derivatives	14,965	8,173
Operating Segments | Retail Natural Gas
Segment Reporting Information [Line Items]
Total Revenues	44,592	58,062
Retail cost of revenues	18,440	29,367
Less:
Net asset optimization (expenses) revenues	0	0
Retail Gross Margin	24,655	26,604
Total Assets	862,873		820,601
Goodwill	2,530		2,530
Operating Segments | Retail Natural Gas | Non-trading
Less:
Net, gain (loss) on non-trading derivative instruments	(147)	2,139
Current period settlements on non-trading derivatives	1,644	(48)
Corporate and Other
Segment Reporting Information [Line Items]
Total Revenues	321	2,552
Retail cost of revenues	0	0
Less:
Net asset optimization (expenses) revenues	321	2,552
Retail Gross Margin	0	0
Total Assets	336,218		341,411
Goodwill	0		0
Corporate and Other | Non-trading
Less:
Net, gain (loss) on non-trading derivative instruments	0	0
Current period settlements on non-trading derivatives	0	0
Eliminations
Segment Reporting Information [Line Items]
Total Revenues	0	0
Retail cost of revenues	0	0
Less:
Net asset optimization (expenses) revenues	0	0
Retail Gross Margin	0	0
Total Assets	(3,431,718)		(3,263,928)
Goodwill	0		$ 0
Eliminations | Non-trading
Less:
Net, gain (loss) on non-trading derivative instruments	0	0
Current period settlements on non-trading derivatives	$ 0	$ 0