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Employee Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Pension        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Interest cost $ 5,331 $ 5,821 $ 10,621 $ 11,626
Expected return on plan assets (5,246) (4,916) (10,445) (9,836)
Amortization of net actuarial loss 316 421 688 799
Net periodic benefit cost 401 1,326 864 2,589
Black Lung        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service cost 625 536 1,249 1,071
Interest cost 1,512 1,463 3,025 2,926
Expected return on plan assets (14) (14) (28) (27)
Amortization of net actuarial loss 1,911 1,076 3,822 2,152
Net periodic benefit cost $ 4,034 $ 3,061 $ 8,068 $ 6,122