Stock-based Compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Feb. 18, 2025
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Number of shares authorized (in shares)	4,290,278
Deferred tax benefit	$ 7,464	$ 2,539	$ 2,244
Unrecognized compensation cost	$ 10,129
Weighted-average recognition period for unrecognized compensation cost (years)	1 year 7 months 28 days
Restricted Stock Units
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Fair value of awards vested	$ 23,323	17,710	8,359
Performance Share Units
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Fair value of awards vested	4,739	1,090	1,161
Selling, General and Administrative Expenses
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock-based compensation expense	$ 32,943	$ 11,350	$ 10,046
Performance Incentive Plan, Remaining Arch Plan Shares
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares authorized (in shares)	1,690,278
Arch Resources, Inc
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Common stock, par value (in dollars per share)				$ 0.01