Revenues - Disaggregation of Revenues (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Retail revenues	$ 166,360	$ 240,154
POR
Disaggregation of Revenue [Line Items]
Retail revenues	107,950	157,916
Non-POR
Disaggregation of Revenue [Line Items]
Retail revenues	58,410	82,238
Unbilled revenue
Disaggregation of Revenue [Line Items]
Retail revenues	(15,763)	(12,811)
Commercial | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	55,532	87,102
Residential | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	126,591	165,863
New England
Disaggregation of Revenue [Line Items]
Retail revenues	53,647	84,762
Mid-Atlantic
Disaggregation of Revenue [Line Items]
Retail revenues	61,646	88,180
Midwest
Disaggregation of Revenue [Line Items]
Retail revenues	28,596	42,596
Southwest
Disaggregation of Revenue [Line Items]
Retail revenues	22,471	24,616
Retail Electricity
Disaggregation of Revenue [Line Items]
Retail revenues	121,768	182,092
Retail Electricity | POR
Disaggregation of Revenue [Line Items]
Retail revenues	84,913	128,937
Retail Electricity | Non-POR
Disaggregation of Revenue [Line Items]
Retail revenues	36,855	53,155
Retail Electricity | Unbilled revenue
Disaggregation of Revenue [Line Items]
Retail revenues	(11,475)	(9,911)
Retail Electricity | Commercial | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	40,015	67,235
Retail Electricity | Residential | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	93,228	124,768
Retail Electricity | New England
Disaggregation of Revenue [Line Items]
Retail revenues	46,593	76,234
Retail Electricity | Mid-Atlantic
Disaggregation of Revenue [Line Items]
Retail revenues	45,842	66,811
Retail Electricity | Midwest
Disaggregation of Revenue [Line Items]
Retail revenues	14,989	22,107
Retail Electricity | Southwest
Disaggregation of Revenue [Line Items]
Retail revenues	14,344	16,940
Retail Natural Gas
Disaggregation of Revenue [Line Items]
Retail revenues	44,592	58,062
Retail Natural Gas | POR
Disaggregation of Revenue [Line Items]
Retail revenues	23,037	28,979
Retail Natural Gas | Non-POR
Disaggregation of Revenue [Line Items]
Retail revenues	21,555	29,083
Retail Natural Gas | Unbilled revenue
Disaggregation of Revenue [Line Items]
Retail revenues	(4,288)	(2,900)
Retail Natural Gas | Commercial | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	15,517	19,867
Retail Natural Gas | Residential | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	33,363	41,095
Retail Natural Gas | New England
Disaggregation of Revenue [Line Items]
Retail revenues	7,054	8,528
Retail Natural Gas | Mid-Atlantic
Disaggregation of Revenue [Line Items]
Retail revenues	15,804	21,369
Retail Natural Gas | Midwest
Disaggregation of Revenue [Line Items]
Retail revenues	13,607	20,489
Retail Natural Gas | Southwest
Disaggregation of Revenue [Line Items]
Retail revenues	$ 8,127	$ 7,676