Derivative Instruments Gains (Loss) on Derivatives Recognized in Income (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Financial Swaps | Off-system sales
Derivative Instruments, Gain (Loss)
Derivative, Loss on Derivative	[1]	$ (305)	$ (51)
Derivative, Gain on Derivative	[1]			$ 1,173	$ 1,395
Financial Swaps | Purchased power
Derivative Instruments, Gain (Loss)
Derivative, Loss on Derivative	[1]	(287)		(735)
Derivative, Gain on Derivative	[1]		164		151
Financial Swaps | Fuel expense
Derivative Instruments, Gain (Loss)
Derivative, Loss on Derivative	[1]	(4)			(2,442)
Derivative, Gain on Derivative	[1]		373	666
Financial Swaps | Other operations and maintenance
Derivative Instruments, Gain (Loss)
Derivative, Loss on Derivative	[1]	(55)	(35)	(81)	(150)
Forward contracts | Purchased power
Derivative Instruments, Gain (Loss)
Derivative, Loss on Derivative	[1]	(8)		(10)
Derivative, Gain on Derivative	[1]		0		0
Forward contracts | Fuel expense
Derivative Instruments, Gain (Loss)
Derivative, Gain on Derivative	[1]	$ 3	$ 93	$ 3	$ 89

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