Disaggregation of Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 157,896	$ 173,822	$ 841,601	$ 898,140
Other	667	541	4,593	4,113
Total operating revenues	158,563	174,363	846,194	902,253
Retail
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	134,277	132,126	744,661	775,174
Wholesale
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	18,540	35,691	75,245	102,221
Wheeling (transmission)
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 5,079	$ 6,005	$ 21,695	$ 20,745