Derivative instruments - Realized and unrealized gains and (losses) on derivative instruments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Gains and (losses) on derivative instruments
Net unrealized losses on derivative instruments	$ 63,994	$ 10,248
Natural gas swaps | Regulatory asset
Gains and (losses) on derivative instruments
Net unrealized losses on derivative instruments	0	10,248
Natural gas swaps | Regulatory liability
Gains and (losses) on derivative instruments
Net unrealized losses on derivative instruments	63,994	0
Natural gas swaps | Fuel
Gains and (losses) on derivative instruments
Gains	31,440	830	$ 224
Losses	(1,431)	(21,179)	(9,308)
Total	$ 30,009	$ (20,349)	$ (9,084)