Derivative instruments - Fair value of derivative instruments not designated as hedging (Details) - Natural gas swaps - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Liabilities:
Liabilities	$ 63,994	$ 10,248
Credit collateral posted	1,800
Not designated as hedges | Other current assets
Assets:
Assets	23,596	222
Not designated as hedges | Other deferred charges
Assets:
Assets	40,398	0
Not designated as hedges | Other current liabilities
Liabilities:
Liabilities	0	2,305
Not designated as hedges | Other deferred credits
Liabilities:
Liabilities	$ 0	$ 8,165