Revenues - Disaggregation of Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Retail revenues	$ 128,618	$ 177,805	$ 294,978	$ 417,959
POR
Disaggregation of Revenue [Line Items]
Retail revenues	82,070	118,198	190,020	280,430
Non-POR
Disaggregation of Revenue [Line Items]
Retail revenues	46,548	59,607	104,958	137,529
Unbilled revenue
Disaggregation of Revenue [Line Items]
Retail revenues	(1,954)	(4,663)	(17,717)	(17,474)
Commercial | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	36,698	68,533	92,230	155,635
Residential | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	93,874	113,935	220,465	279,798
New England
Disaggregation of Revenue [Line Items]
Retail revenues	42,890	71,057	96,537	155,819
Mid-Atlantic
Disaggregation of Revenue [Line Items]
Retail revenues	44,672	59,837	106,318	148,017
Midwest
Disaggregation of Revenue [Line Items]
Retail revenues	18,697	22,209	47,293	64,805
Southwest
Disaggregation of Revenue [Line Items]
Retail revenues	22,359	24,702	44,830	49,318
Retail Electricity
Disaggregation of Revenue [Line Items]
Retail revenues	112,255	160,776	234,023	342,868
Retail Electricity | POR
Disaggregation of Revenue [Line Items]
Retail revenues	73,694	110,270	158,607	239,207
Retail Electricity | Non-POR
Disaggregation of Revenue [Line Items]
Retail revenues	38,561	50,506	75,416	103,661
Retail Electricity | Unbilled revenue
Disaggregation of Revenue [Line Items]
Retail revenues	3,938	3,658	(7,537)	(6,253)
Retail Electricity | Commercial | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	29,408	59,699	69,423	126,934
Retail Electricity | Residential | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	78,909	97,419	172,137	222,187
Retail Electricity | New England
Disaggregation of Revenue [Line Items]
Retail revenues	40,161	67,905	86,754	144,139
Retail Electricity | Mid-Atlantic
Disaggregation of Revenue [Line Items]
Retail revenues	38,743	54,503	84,585	121,314
Retail Electricity | Midwest
Disaggregation of Revenue [Line Items]
Retail revenues	14,506	17,473	29,495	39,580
Retail Electricity | Southwest
Disaggregation of Revenue [Line Items]
Retail revenues	18,845	20,895	33,189	37,835
Retail Natural Gas
Disaggregation of Revenue [Line Items]
Retail revenues	16,363	17,029	60,955	75,091
Retail Natural Gas | POR
Disaggregation of Revenue [Line Items]
Retail revenues	8,376	7,928	31,413	41,223
Retail Natural Gas | Non-POR
Disaggregation of Revenue [Line Items]
Retail revenues	7,987	9,101	29,542	33,868
Retail Natural Gas | Unbilled revenue
Disaggregation of Revenue [Line Items]
Retail revenues	(5,892)	(8,321)	(10,180)	(11,221)
Retail Natural Gas | Commercial | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	7,290	8,834	22,807	28,701
Retail Natural Gas | Residential | Billed revenue
Disaggregation of Revenue [Line Items]
Retail revenues	14,965	16,516	48,328	57,611
Retail Natural Gas | New England
Disaggregation of Revenue [Line Items]
Retail revenues	2,729	3,152	9,783	11,680
Retail Natural Gas | Mid-Atlantic
Disaggregation of Revenue [Line Items]
Retail revenues	5,929	5,334	21,733	26,703
Retail Natural Gas | Midwest
Disaggregation of Revenue [Line Items]
Retail revenues	4,191	4,736	17,798	25,225
Retail Natural Gas | Southwest
Disaggregation of Revenue [Line Items]
Retail revenues	$ 3,514	$ 3,807	$ 11,641	$ 11,483