Derivative Instruments Gains (Loss) on Derivatives Recognized in Income (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2021	[1]	Jun. 30, 2020	[1]	Jun. 30, 2021	Jun. 30, 2020
Financial Swaps | Operating revenues
Derivative Instruments, Gain (Loss)
Derivative, Gain on Derivative	$ 0		$ 308		$ 0	$ 1,134
Financial Swaps | Purchased power
Derivative Instruments, Gain (Loss)
Derivative, Loss on Derivative			1,125			1,315
Derivative, Gain on Derivative	0				249
Financial Swaps | Operating Expense
Derivative Instruments, Gain (Loss)
Derivative, Loss on Derivative			69			2,917
Derivative, Gain on Derivative	903				1,636
Forward contracts | Operating revenues
Derivative Instruments, Gain (Loss)
Derivative, Gain on Derivative	26		41		73	120
Forward contracts | Purchased power
Derivative Instruments, Gain (Loss)
Derivative, Loss on Derivative	26		39		73	115
Forward contracts | Operating Expense
Derivative Instruments, Gain (Loss)
Derivative, Loss on Derivative			$ 1			$ 20
Derivative, Gain on Derivative	$ 2				$ 1

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