Derivative Instruments Gains (Loss) on Derivatives Recognized in Income (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2021	[1]	Sep. 30, 2020	[1]	Sep. 30, 2021	Sep. 30, 2020
Financial Swaps | Operating revenues
Derivative Instruments, Gain (Loss)
Derivative, Gain on Derivative	$ (361)		$ (1,039)		$ (361)	$ (2,173)
Financial Swaps | Purchased power
Derivative Instruments, Gain (Loss)
Derivative, Loss on Derivative			2,264			3,579
Derivative, Gain on Derivative	(2,194)				(2,443)
Financial Swaps | Operating Expense
Derivative Instruments, Gain (Loss)
Derivative, Loss on Derivative			1,115			4,032
Derivative, Gain on Derivative	(9,011)				(10,647)
Forward contracts | Operating revenues
Derivative Instruments, Gain (Loss)
Derivative, Gain on Derivative	(1,709)		(229)		(1,783)	(349)
Forward contracts | Purchased power
Derivative Instruments, Gain (Loss)
Derivative, Loss on Derivative	842		198		916	313
Forward contracts | Operating Expense
Derivative Instruments, Gain (Loss)
Derivative, Loss on Derivative	$ 132		$ 6		$ 132	$ 25

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