Management - Labor Relations (Details)	12 Months Ended
	Dec. 31, 2020 numberOfEmployee numberOfMine
Concentration Risk [Line Items]
Number of employees	4,600
Number of hourly employees	3,500
Hourly employees represented by organized labor unions (in percent)	29.00%
Coal production generated by hourly employees represented by organized labor unions (in percent)	18.00%
Number of us mines represented by unions | numberOfMine	1
Period of agreement with UMWA covering hourly coworkers involved in mining reclamation	4 years
Wilpinjong Employees
Concentration Risk [Line Items]
Australian hourly employees under contract negotiations (in percent)	39.00%
Coal production generated by Australian hourly employees under contract negotiation (in percent)	59.00%
Moorvale Employees
Concentration Risk [Line Items]
Australian hourly employees under contract negotiations (in percent)	15.00%
Coal production generated by Australian hourly employees under contract negotiation (in percent)	5.00%
Metropolitan Employees
Concentration Risk [Line Items]
Australian hourly employees under contract negotiations (in percent)	14.00%
Coal production generated by Australian hourly employees under contract negotiation (in percent)	4.00%
Wambo Operation Employees
Concentration Risk [Line Items]
Australian hourly employees under contract negotiations (in percent)	8.00%
Coal production generated by Australian hourly employees under contract negotiation (in percent)	23.00%
Coppabella/Moorvale Employees
Concentration Risk [Line Items]
Australian hourly employees under contract negotiations (in percent)	23.00%
Coal production generated by Australian hourly employees under contract negotiation (in percent)	9.00%
Shoal Creek
Concentration Risk [Line Items]
Coal production generated by hourly employees (in percent)	1.00%