DERIVATIVE FINANCIAL INSTRUMENTS: Derivative Instruments, Gain (Loss) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financial Swaps | Operating revenues
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative	[1]	$ 904	$ 1,316	$ 902
Financial Swaps | Purchased power
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative	[1]		7,828	166
Derivative, Loss on Derivative	[1]	(2,183)
Financial Swaps | Fuel Expense
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative	[1]	13,811	22,563	701
Financial Swaps | Other operations and maintenance
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative	[1]	0	118
Derivative, Loss on Derivative	[1]			(84)
Forward contracts | Operating revenues
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative	[1]	285	41	55
Forward contracts | Purchased power
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Loss on Derivative	[1]	(270)	(54)	(69)
Forward contracts | Fuel Expense
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative	[1]	$ 565		$ 4
Derivative, Loss on Derivative	[1]		$ (186)

[1] Excludes unrealized gains or losses on derivatives, which are recorded on the balance sheet as regulatory assets or regulatory liabilities.