Management - Labor Relations Labor Relations (Details)	12 Months Ended
	Apr. 30, 2016 Numberofmonths	Dec. 31, 2015
Concentration Risk [Line Items]
Entity Number of Employees		7,600
Entity Number Of Hourly Employees		5,700
Percentage of hourly employees represented by organized labor unions		37.00%
Percentage Of Coal Production Generated By Hourly Employees Represented By Organized Labor Unions		20.00%
Number of US Mines Represented by Unions		1
Percentage Of Australian Hourly Employees Under Contract Negotiations		28.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation		19.00%
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 Underground [Member]
Concentration Risk [Line Items]
Enterprise agreement rollover months	12