Derivatives - Effects of derivatives on measures of financial performance (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
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	$ 46	$ 12,252	$ (1,361)	$ (5,144)
Coal Contract | Other operating (income) expense, net
Derivative Instruments, Gain (Loss) [Line Items]
Realized Gain (Loss) on Derivatives		(14,700)	6,254	(41,159)
Heating oil - diesel purchases | Other operating (income) expense, net
Derivative Instruments, Gain (Loss) [Line Items]
Heating oil realized and unrealized gains and losses	$ 1,341	$ (4,146)	$ (1,037)	$ 9,386