Disaggregation of Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 202,449	$ 236,084	$ 376,271	$ 411,141	$ 864,505	$ 901,864
Other	626	712	1,167	1,368	4,027	4,264
Total operating revenues	203,075	236,796	377,438	412,509	868,532	906,128
Retail
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	183,659	220,980	315,785	367,607	737,854	812,522
Wholesale
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	13,939	10,957	49,630	35,101	105,202	71,610
Wheeling (transmission)
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 4,851	$ 4,147	$ 10,856	$ 8,433	$ 21,449	$ 17,732