Derivative Instruments - Realized and unrealized gains and (losses) on derivative instruments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Gains and (losses) on derivative instruments
Net unrealized losses on derivative instruments	$ 32,256	$ 13,154
Natural gas swaps | Regulatory asset
Gains and (losses) on derivative instruments
Net unrealized losses on derivative instruments	32,256	13,154
Natural gas swaps | Fuel
Gains and (losses) on derivative instruments
Gains	224	6,088	$ 3,818
Losses	(9,308)	(956)	(1,677)
Total	$ (9,084)	$ 5,132	$ 2,141