Commitments and Contingencies - Narrative (Details) T in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023 USD ($) T	Jun. 30, 2022 USD ($) T	Jun. 30, 2023 USD ($) T	Jun. 30, 2022 USD ($) T	Jan. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Feb. 10, 2022 USD ($)	Feb. 21, 2020 USD ($)	Feb. 20, 2020 USD ($)
Long-term Purchase Commitment [Line Items]
Coal royalty expense	$ 45,523	$ 72,627	$ 97,024	$ 129,970
Black Lung
Long-term Purchase Commitment [Line Items]
Collateral for black lung obligations							$ 10,000	$ 65,700	$ 2,600
Black Lung | Minimum
Long-term Purchase Commitment [Line Items]
Collateral for black lung obligations					$ 80,000
Black Lung | Maximum
Long-term Purchase Commitment [Line Items]
Collateral for black lung obligations					$ 100,000
Surety Bond
Long-term Purchase Commitment [Line Items]
Outstanding surety bonds	164,997		164,997
Reclamation-Related Obligations | Collateral Pledged
Long-term Purchase Commitment [Line Items]
Restricted cash, restricted investments, and deposits	35,416		35,416
New ABL Facility | Line of Credit
Long-term Purchase Commitment [Line Items]
Letters of credit outstanding	61,927		61,927			$ 61,877
Revolving Credit Facility | New ABL Facility | Line of Credit
Long-term Purchase Commitment [Line Items]
Letters of credit outstanding	$ 61,927		$ 61,927
Cumberland Back-to-Back Coal Supply Agreements
Long-term Purchase Commitment [Line Items]
Purchase and sale contingency, mass purchased and sold (in tons) | T	206	258	399	677
Purchase and sale contingency, mass purchased and sold, amount	$ 7,829	$ 9,936	$ 15,170	$ 26,121