REVENUES: Receivables and Allowance for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Receivables and Allowance for Uncollectible Accounts [Line Items]
Accounts Receivable, Allowance for Credit Loss	$ 4,252	$ 3,501	$ 4,252	$ 3,501	$ 4,766	$ 1,401
Additions to the allowance			369	2,786
Write-offs, net of recoveries			$ (883)	$ (686)
Allowance for uncollectible accounts as a percentage of customer receivables	4.80%	4.30%	4.80%	4.30%
Alternative revenue programs and other revenues	$ 1,597	$ 6,695	$ 25,838	$ 26,999
Idaho fixed cost adjustment mechanism
Receivables and Allowance for Uncollectible Accounts [Line Items]
Alternative revenue programs and other revenues	(550)	4,532	15,754	20,725
Derivative revenues
Receivables and Allowance for Uncollectible Accounts [Line Items]
Alternative revenue programs and other revenues	$ 2,147	$ 2,163	$ 10,084	$ 6,274