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Management - Labor Relations Labor Relations (Details)	12 Months Ended
	Dec. 31, 2016
Concentration Risk [Line Items]
Entity Number of Employees	6,700
Entity Number Of Hourly Employees	5,100
Percentage of hourly employees represented by organized labor unions	39.00%
Percentage Of Coal Production Generated By Hourly Employees Represented By Organized Labor Unions	22.00%
Number of US Mines Represented by Unions	1
United Mine Workers of America [Member]
Concentration Risk [Line Items]
Percentage of hourly employees represented by organized labor unions	8.00%
Percentage Of Coal Production Generated By Hourly Employees	4.00%
North Wambo UG Employees [Domain]
Concentration Risk [Line Items]
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation	10.00%
North Wambo UG Employees [Member]
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations	8.00%
Metropolitan Employees [Member]
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations	11.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation	6.00%
Millenium 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	11.00%
Wilpinjong Employees [Member]
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations	18.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation	42.00%
Coppabella/Moorvale Employees [Member]
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations	28.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation	13.00%

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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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