Derivatives - Effects of derivatives on measures of financial performance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Derivative Instruments, Gain (Loss) [Line Items]
Change in fair value of coal derivatives and coal trading activities, net total	$ (1,572)	$ (1,274)	$ 2,392
Not Designated as Hedging Instrument | Coal Contract | Change in fair value of coal derivatives and coal trading activities, net
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives	(1,572)	(1,274)	2,392
Not Designated as Hedging Instrument | Coal Contract | Other operating (income) expense, net
Derivative Instruments, Gain (Loss) [Line Items]
Realized Gain (Loss) on Derivatives	6,254	(42,068)	$ (27,464)
Not Designated as Hedging Instrument | Heating oil - diesel purchases | Other operating (income) expense, net
Derivative Instruments, Gain (Loss) [Line Items]
Heating oil realized and unrealized gains and losses	$ (4,079)	$ 8,309