Derivative Instruments - Realized and unrealized gains and (losses) on derivative instruments (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Gains and (losses) on derivative instruments
Net unrealized losses on derivative instruments			$ 31,929	$ 32,256
Natural gas swaps \| Regulatory asset
Gains and (losses) on derivative instruments
Net unrealized losses on derivative instruments			31,929	32,256
Natural gas swaps \| Fuel
Gains and (losses) on derivative instruments
Gains	$ 0	$ 11	0	224
Losses	(7,141)	(1,126)	(11,593)	(1,799)
Total	$ (7,141)	$ (1,115)	$ (11,593)	$ (1,575)

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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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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