Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Revenue	$ 298,080	$ 288,433	$ 267,632
Operating expenses:
Fuel and purchased power	92,886	87,638	75,026
Operations and maintenance	79,523	74,064	66,384
Depreciation and amortization	39,649	35,862	34,030
Taxes - property	7,687	7,043	6,612
Total operating expenses	219,745	204,607	182,052
Operating income	78,335	83,826	85,580
Other income (expense):
Interest expense	(22,545)	(22,421)	(22,908)
AFUDC - borrowed	521	1,137	1,140
Interest income	676	904	1,576
AFUDC - equity	221	2,165	2,165
Other income (expense), net	(891)	(185)	113
Total other income (expense)	(22,018)	(18,400)	(17,914)
Income before income taxes	56,317	65,426	67,666
Income tax expense	(10,672)	(14,128)	(22,528)
Net income	$ 45,645	$ 51,298	$ 45,138