Derivative Instruments - Realized and unrealized gains and (losses) on derivative instruments (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Gains and (losses) on derivative instruments
Net unrealized losses on derivative instruments	$ 1,862		$ 10,248
Natural gas swaps \| Regulatory asset
Gains and (losses) on derivative instruments
Net unrealized losses on derivative instruments	1,862		$ 10,248
Natural gas swaps \| Fuel
Gains and (losses) on derivative instruments
Gains	101	$ 0
Losses	(1,244)	(4,452)
Total	$ (1,143)	$ (4,452)

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

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Amount of increase in the fair value of derivatives recognized in the income statement.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amount of decrease in the fair value of derivatives recognized in the income statement.

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

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