Derivative Instruments Gains (Loss) on Derivatives Recognized in Income (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Financial Swaps | Operating revenues
Derivative Instruments, Gain (Loss)
Derivative, Loss on Derivative	[1]			$ 2,789
Derivative, Gain on Derivative	[1]	$ 414	$ 27		$ 266
Financial Swaps | Purchased power
Derivative Instruments, Gain (Loss)
Derivative, Loss on Derivative	[1]	56			189
Derivative, Gain on Derivative	[1]		13	687
Financial Swaps | Fuel expense
Derivative Instruments, Gain (Loss)
Derivative, Loss on Derivative	[1]		112		800
Derivative, Gain on Derivative	[1]	7		12,402
Financial Swaps | Other operations and maintenance
Derivative Instruments, Gain (Loss)
Derivative, Gain on Derivative	[1]	0	31	0	38
Forward contracts | Operating revenues
Derivative Instruments, Gain (Loss)
Derivative, Gain on Derivative	[1]	0	0	64	2
Forward contracts | Purchased power
Derivative Instruments, Gain (Loss)
Derivative, Loss on Derivative	[1]		7	64	20
Derivative, Gain on Derivative	[1]	0
Forward contracts | Fuel expense
Derivative Instruments, Gain (Loss)
Derivative, Gain on Derivative	[1]	$ 0	$ 10	$ 416	$ 24

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