Disaggregation of Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 293,158	$ 299,369	$ 669,429	$ 710,510	$ 858,294	$ 905,063
Other	1,295	902	2,462	2,270	4,420	3,866
Total operating revenues	294,453	300,271	671,891	712,780	862,714	908,929
Retail
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	268,375	265,971	584,160	633,578	740,258	807,626
Wholesale
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	18,900	28,253	68,530	63,354	95,849	79,456
Wheeling (transmission)
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 5,883	$ 5,145	$ 16,739	$ 13,578	$ 22,187	$ 17,981