DERIVATIVE FINANCIAL INSTRUMENTS: Derivative Instruments, Gain (Loss) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Energy Related Derivative | Off-system sales
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative	[1]	$ 2,882
Derivative, Loss on Derivative	[1]		$ (4,119)	$ (2,637)
Energy Related Derivative | Purchased power
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative	[1]	748		947
Derivative, Loss on Derivative	[1]		(1,416)
Energy Related Derivative | Fuel Expense
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative	[1]		3,862	731
Derivative, Loss on Derivative	[1]	(6,045)
Energy Related Derivative | Operating Expense
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative	[1]			35
Derivative, Loss on Derivative	[1]	(50)	(158)
Forward contracts | Off-system sales
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative	[1]	0	277	185
Forward contracts | Purchased power
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Loss on Derivative	[1]	(6)	(279)	(196)
Forward contracts | Fuel Expense
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative	[1]	$ 54	$ 94	$ 217

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