Revenues - Disaggregation of Revenues (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Retail revenues	$ 113,145	$ 166,360
POR
Disaggregation of Revenue [Line Items]
Retail revenues	70,450	107,950
Non-POR
Disaggregation of Revenue [Line Items]
Retail revenues	42,695	58,410
Unbilled revenue
Disaggregation of Revenue [Line Items]
Retail revenues	(13,349)	(15,763)
Commercial | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	26,732	55,532
Residential | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	99,762	126,591
New England
Disaggregation of Revenue [Line Items]
Retail revenues	30,618	53,647
Mid-Atlantic
Disaggregation of Revenue [Line Items]
Retail revenues	42,005	61,646
Midwest
Disaggregation of Revenue [Line Items]
Retail revenues	21,297	28,596
Southwest
Disaggregation of Revenue [Line Items]
Retail revenues	19,225	22,471
Retail Electricity
Disaggregation of Revenue [Line Items]
Retail revenues	78,755	121,768
Retail Electricity | POR
Disaggregation of Revenue [Line Items]
Retail revenues	50,850	84,913
Retail Electricity | Non-POR
Disaggregation of Revenue [Line Items]
Retail revenues	27,905	36,855
Retail Electricity | Unbilled revenue
Disaggregation of Revenue [Line Items]
Retail revenues	(9,733)	(11,475)
Retail Electricity | Commercial | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	15,216	40,015
Retail Electricity | Residential | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	73,272	93,228
Retail Electricity | New England
Disaggregation of Revenue [Line Items]
Retail revenues	26,241	46,593
Retail Electricity | Mid-Atlantic
Disaggregation of Revenue [Line Items]
Retail revenues	28,550	45,842
Retail Electricity | Midwest
Disaggregation of Revenue [Line Items]
Retail revenues	11,059	14,989
Retail Electricity | Southwest
Disaggregation of Revenue [Line Items]
Retail revenues	12,905	14,344
Retail Natural Gas
Disaggregation of Revenue [Line Items]
Retail revenues	34,390	44,592
Retail Natural Gas | POR
Disaggregation of Revenue [Line Items]
Retail revenues	19,600	23,037
Retail Natural Gas | Non-POR
Disaggregation of Revenue [Line Items]
Retail revenues	14,790	21,555
Retail Natural Gas | Unbilled revenue
Disaggregation of Revenue [Line Items]
Retail revenues	(3,616)	(4,288)
Retail Natural Gas | Commercial | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	11,516	15,517
Retail Natural Gas | Residential | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	26,490	33,363
Retail Natural Gas | New England
Disaggregation of Revenue [Line Items]
Retail revenues	4,377	7,054
Retail Natural Gas | Mid-Atlantic
Disaggregation of Revenue [Line Items]
Retail revenues	13,455	15,804
Retail Natural Gas | Midwest
Disaggregation of Revenue [Line Items]
Retail revenues	10,238	13,607
Retail Natural Gas | Southwest
Disaggregation of Revenue [Line Items]
Retail revenues	$ 6,320	$ 8,127