Revenues - Disaggregation of Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Retail revenues	$ 141,188	$ 207,341	$ 436,166	$ 625,300
POR
Disaggregation of Revenue [Line Items]
Retail revenues	90,449	140,720	280,469	421,150
Non-POR
Disaggregation of Revenue [Line Items]
Retail revenues	50,739	66,621	155,697	204,150
Unbilled revenue
Disaggregation of Revenue [Line Items]
Retail revenues	(3,871)	(6,577)	(21,588)	(24,051)
Commercial | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	36,885	72,459	129,115	228,094
Residential | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	108,174	141,459	328,639	421,257
New England
Disaggregation of Revenue [Line Items]
Retail revenues	47,825	78,626	144,362	234,445
Mid-Atlantic
Disaggregation of Revenue [Line Items]
Retail revenues	49,121	72,703	155,439	220,720
Midwest
Disaggregation of Revenue [Line Items]
Retail revenues	18,473	25,456	65,766	90,261
Southwest
Disaggregation of Revenue [Line Items]
Retail revenues	25,769	30,556	70,599	79,874
Retail Electricity
Disaggregation of Revenue [Line Items]
Retail revenues	132,958	197,010	366,981	539,878
Retail Electricity | POR
Disaggregation of Revenue [Line Items]
Retail revenues	87,439	136,683	246,046	375,890
Retail Electricity | Non-POR
Disaggregation of Revenue [Line Items]
Retail revenues	45,519	60,327	120,935	163,988
Retail Electricity | Unbilled revenue
Disaggregation of Revenue [Line Items]
Retail revenues	(4,314)	(6,834)	(11,851)	(13,087)
Retail Electricity | Commercial | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	34,180	69,081	103,603	196,015
Retail Electricity | Residential | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	103,092	134,763	275,229	356,950
Retail Electricity | New England
Disaggregation of Revenue [Line Items]
Retail revenues	46,464	76,995	133,218	221,134
Retail Electricity | Mid-Atlantic
Disaggregation of Revenue [Line Items]
Retail revenues	46,878	69,763	131,463	191,077
Retail Electricity | Midwest
Disaggregation of Revenue [Line Items]
Retail revenues	16,933	23,310	46,428	62,890
Retail Electricity | Southwest
Disaggregation of Revenue [Line Items]
Retail revenues	22,683	26,942	55,872	64,777
Retail Natural Gas
Disaggregation of Revenue [Line Items]
Retail revenues	8,230	10,331	69,185	85,422
Retail Natural Gas | POR
Disaggregation of Revenue [Line Items]
Retail revenues	3,010	4,037	34,423	45,260
Retail Natural Gas | Non-POR
Disaggregation of Revenue [Line Items]
Retail revenues	5,220	6,294	34,762	40,162
Retail Natural Gas | Unbilled revenue
Disaggregation of Revenue [Line Items]
Retail revenues	443	257	(9,737)	(10,964)
Retail Natural Gas | Commercial | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	2,705	3,378	25,512	32,079
Retail Natural Gas | Residential | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	5,082	6,696	53,410	64,307
Retail Natural Gas | New England
Disaggregation of Revenue [Line Items]
Retail revenues	1,361	1,631	11,144	13,311
Retail Natural Gas | Mid-Atlantic
Disaggregation of Revenue [Line Items]
Retail revenues	2,243	2,940	23,976	29,643
Retail Natural Gas | Midwest
Disaggregation of Revenue [Line Items]
Retail revenues	1,540	2,146	19,338	27,371
Retail Natural Gas | Southwest
Disaggregation of Revenue [Line Items]
Retail revenues	$ 3,086	$ 3,614	$ 14,727	$ 15,097