REVENUES: Alternative Revenue Program and Other Revenues (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Alternative Revenue Program and Other Revenues [Line Items]
Alternative revenue programs and other revenues	$ 57,654	$ 54,470	$ 24,889
Idaho fixed cost adjustment mechanism
Alternative Revenue Program and Other Revenues [Line Items]
Alternative revenue programs and other revenues	36,923	35,924	18,196
Derivative revenues
Alternative Revenue Program and Other Revenues [Line Items]
Alternative revenue programs and other revenues	$ 20,731	$ 18,546	$ 6,693