Management - Labor Relations Labor Relations (Details)	12 Months Ended
Dec. 31, 2019
Concentration Risk [Line Items]
Entity Number of Employees	6,600
Entity Number Of Hourly Employees	5,000
Percentage of hourly employees represented by organized labor unions	42.00%
Percentage Of Coal Production Generated By Hourly Employees Represented By Organized Labor Unions	19.00%
Number of US Mines Represented by Unions	2
Millenium Employees
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations	1.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation	2.00%
Wilpinjong Employees
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations	25.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation	54.00%
Moorvale Employees
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations	14.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation	6.00%
Metropolitan Employees
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations	14.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation	6.00%
Wambo Operation Employees
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations	24.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation	22.00%
Coppabella/Moorvale Employees
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations	22.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation	9.00%
Kayenta Mine \| United Mine Workers of America
Concentration Risk [Line Items]
Percentage Of Coal Production Generated By Hourly Employees	3.00%
Shoal Creek
Concentration Risk [Line Items]
Percentage Of Coal Production Generated By Hourly Employees	1.00%
Shoal Creek \| United Mine Workers of America
Concentration Risk [Line Items]
Percentage of hourly employees represented by organized labor unions	11.00%

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Number of US Mines Represented by Unions

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Percentage of Australian hourly employees under contract negotiations.

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Percentage of coal production generated by hourly Australian employees under contract negotiation

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Percentage Of Coal Production Generated By Hourly Employees Represented By Organized Labor Unions

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Percentage of coal production generated by hourly employees in the U.S.

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Percentage of employees represented by organized labor unions.

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Number of persons employed by the Entity

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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