Segment Reporting - Financial Data for Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Total Revenues	$ 82,195	$ 128,536	$ 195,200	$ 295,217
Retail cost of revenues	36,176	65,605	158,344	184,428
Less:
Net asset optimization (expense) revenue	(114)	(82)	(254)	239
Net, gain (loss) on non-trading derivative instruments	18,904	8,121	25,928	(16,466)
Current period settlements on non-trading derivatives	795	9,964	(390)	26,572
Non-recurring event - Winter Storm Uri	0	0	(64,900)	0
Retail Gross Margin	26,440	44,994	76,452	100,455
Total assets	369,154		369,154		$ 366,667
Goodwill	120,343		120,343		120,343
Non-trading
Less:
Net, gain (loss) on non-trading derivative instruments	18,898	7,964	25,952	(16,569)
Current period settlements on non-trading derivatives	795	10,055	(394)	26,664
Operating Segments | Retail Electricity
Segment Reporting Information [Line Items]
Total Revenues	71,689	112,255	150,444	234,023
Retail cost of revenues	31,203	59,268	138,727	159,651
Less:
Net asset optimization (expense) revenue	0	0	0	0
Non-recurring event - Winter Storm Uri			(64,900)
Retail Gross Margin	21,651	35,573	52,265	66,379
Total assets	3,051,393		3,051,393		2,906,139
Goodwill	117,813		117,813		117,813
Operating Segments | Retail Electricity | Non-trading
Less:
Net, gain (loss) on non-trading derivative instruments	17,694	8,047	24,399	(16,339)
Current period settlements on non-trading derivatives	1,141	9,367	(47)	24,332
Operating Segments | Retail Natural Gas
Segment Reporting Information [Line Items]
Total Revenues	10,620	16,363	45,010	60,955
Retail cost of revenues	4,973	6,337	19,617	24,777
Less:
Net asset optimization (expense) revenue	0	0	0	0
Non-recurring event - Winter Storm Uri			0
Retail Gross Margin	4,789	9,421	24,187	34,076
Total assets	978,072		978,072		941,569
Goodwill	2,530		2,530		2,530
Operating Segments | Retail Natural Gas | Non-trading
Less:
Net, gain (loss) on non-trading derivative instruments	1,204	(83)	1,553	(230)
Current period settlements on non-trading derivatives	(346)	688	(347)	2,332
Corporate and Other
Segment Reporting Information [Line Items]
Total Revenues	(114)	(82)	(254)	239
Retail cost of revenues	0	0	0	0
Less:
Net asset optimization (expense) revenue	(114)	(82)	(254)	239
Non-recurring event - Winter Storm Uri			0
Retail Gross Margin	0	0	0	0
Total assets	336,135		336,135		318,865
Goodwill	0		0		0
Corporate and Other | Non-trading
Less:
Net, gain (loss) on non-trading derivative instruments	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) revenue	0	0	0	0
Non-recurring event - Winter Storm Uri			0
Retail Gross Margin	0	0	0	0
Total assets	(3,996,446)		(3,996,446)		(3,799,906)
Goodwill	0		0		$ 0
Eliminations | Non-trading
Less:
Net, gain (loss) on non-trading derivative instruments	0	0	0	0
Current period settlements on non-trading derivatives	$ 0	$ 0	$ 0	$ 0