Fair Value of Financial Instruments and Fair Value Measurements - Acquisition-related Obligations at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	$ 29,315	$ 40,450
Carrying Amount | Retiree Committee VEBA Funding Settlement Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	956	3,337
Carrying Amount | UMWA Funds Settlement Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	4,870	4,239
Carrying Amount | Reclamation Funding Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	10,321	18,106
Carrying Amount | Environmental Settlement Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	13,168	14,768
Fair Value, Measurements, Nonrecurring | Total Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	30,105	42,418
Fair Value, Measurements, Nonrecurring | Total Fair Value | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	0	0
Fair Value, Measurements, Nonrecurring | Total Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	0	0
Fair Value, Measurements, Nonrecurring | Total Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	30,105	42,418
Fair Value, Measurements, Nonrecurring | Total Fair Value | Retiree Committee VEBA Funding Settlement Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	969	3,391
Fair Value, Measurements, Nonrecurring | Total Fair Value | Retiree Committee VEBA Funding Settlement Liability | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	0	0
Fair Value, Measurements, Nonrecurring | Total Fair Value | Retiree Committee VEBA Funding Settlement Liability | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	0	0
Fair Value, Measurements, Nonrecurring | Total Fair Value | Retiree Committee VEBA Funding Settlement Liability | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	969	3,391
Fair Value, Measurements, Nonrecurring | Total Fair Value | UMWA Funds Settlement Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	5,134	4,729
Fair Value, Measurements, Nonrecurring | Total Fair Value | UMWA Funds Settlement Liability | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	0	0
Fair Value, Measurements, Nonrecurring | Total Fair Value | UMWA Funds Settlement Liability | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	0	0
Fair Value, Measurements, Nonrecurring | Total Fair Value | UMWA Funds Settlement Liability | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	5,134	4,729
Fair Value, Measurements, Nonrecurring | Total Fair Value | Reclamation Funding Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	10,814	19,362
Fair Value, Measurements, Nonrecurring | Total Fair Value | Reclamation Funding Liability | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	0	0
Fair Value, Measurements, Nonrecurring | Total Fair Value | Reclamation Funding Liability | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	0	0
Fair Value, Measurements, Nonrecurring | Total Fair Value | Reclamation Funding Liability | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	10,814	19,362
Fair Value, Measurements, Nonrecurring | Total Fair Value | Environmental Settlement Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	13,188	14,936
Fair Value, Measurements, Nonrecurring | Total Fair Value | Environmental Settlement Obligations | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	0	0
Fair Value, Measurements, Nonrecurring | Total Fair Value | Environmental Settlement Obligations | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	0	0
Fair Value, Measurements, Nonrecurring | Total Fair Value | Environmental Settlement Obligations | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Fair Value	$ 13,188	$ 14,936