REVENUES: Electric utility operating revenues (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Cumulative Effect of New Accounting Principle in Period of Adoption				$ 0
Revenue from contracts with customers	$ 1,285,286,000	$ 1,312,112,000	$ 1,320,004,000
Alternative revenue programs and other revenues	57,654,000	54,470,000	24,889,000
Electric utility revenues	$ 1,342,940,000	$ 1,366,582,000	$ 1,344,893,000