Management - Labor Relations Labor Relations (Details)	Dec. 31, 2012
Concentration Risk [Line Items]
Entity Number of Employees	8,200
Entity Number Of Hourly Employees	5,700
Percentage of employees represented by organized labor unions	30.00%
Percentage Of Coal Production Generated By Hourly Employees Represented By Organized Labor Unions	19.00%
Number of US Mines Represented by Unions	2
United Mine Workers of America [Member]
Concentration Risk [Line Items]
Percentage of employees represented by organized labor unions	7.00%
Percentage Of Coal Production Generated By Hourly Employees	4.00%