Disaggregation of Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 173,822	$ 175,057	$ 898,140	$ 916,922
Other	541	656	4,113	4,253
Total operating revenues	174,363	175,713	902,253	921,175
Retail
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	132,126	146,628	775,174	826,148
Wholesale
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	35,691	24,143	102,221	72,641
Wheeling (transmission)
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 6,005	$ 4,286	$ 20,745	$ 18,133