Derivative instruments - Fair value of derivative instruments not designated as hedging (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Liabilities:
Derivative Liability, Statement of Financial Position [Extensible Enumeration]	Other current liabilities
Natural gas swaps
Assets:
Assets	$ 131,804	$ 63,994
Not designated as hedges | Natural gas swaps
Liabilities:
Liabilities	3,206	0
Not designated as hedges | Natural gas swaps | Other current assets
Assets:
Assets	35,285	23,596
Not designated as hedges | Natural gas swaps | Other deferred charges
Assets:
Assets	$ 99,725	$ 40,398