Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Revenue	$ 277,864	$ 268,488	$ 254,027
Operating expenses:
Fuel and purchased power	83,339	93,976	89,437
Operations and maintenance	68,088	70,356	68,857
Depreciation and amortization	32,552	29,100	28,125
Taxes - property	5,971	5,942	5,264
Total operating expenses	189,950	199,374	191,683
Operating income	87,914	69,114	62,344
Other income (expense):
Interest expense	(22,337)	(20,569)	(19,725)
AFUDC - borrowed	506	248	186
Interest income	657	619	248
AFUDC - equity	918	519	368
Other expense	(117)	(105)	(196)
Other income	233	248	367
Total other income (expense)	(20,140)	(19,040)	(18,752)
Income from continuing operations before income taxes	67,774	50,074	43,592
Income tax (expense) benefit	(22,600)	(16,512)	(13,419)
Net income	$ 45,174	$ 33,562	$ 30,173