Management - Labor Relations Labor Relations (Details)	3 Months Ended	9 Months Ended
Apr. 01, 2017	Dec. 31, 2017
Concentration Risk [Line Items]
Entity Number of Employees		7,100
Entity Number Of Hourly Employees		5,500
Percentage of hourly employees represented by organized labor unions		38.00%
Percentage Of Coal Production Generated By Hourly Employees Represented By Organized Labor Unions	20.00%
Number of US Mines Represented by Unions		1
Successor \| United Mine Workers of America [Member]
Concentration Risk [Line Items]
Percentage of hourly employees represented by organized labor unions		7.00%
Percentage Of Coal Production Generated By Hourly Employees	4.00%
Successor \| North Wambo UG Employees [Member]
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations		20.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation	19.00%
Successor \| North Goonyella UG Employees [Member]
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations		7.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation		11.00%
Successor \| Metropolitan Employees [Member]
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations		12.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation		3.00%
Successor \| Coppabella/Moorvale Employees [Member]
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations		16.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation	9.00%
Successor \| Millenium Employees [Member]
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations		15.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation		10.00%
Successor \| Wilpinjong Employees [Member]
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations		21.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation		42.00%

Entity number of hourly employees.

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- Definition

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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