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Related Party Transactions (Details 1) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2010
Sep. 30, 2010
Jun. 30, 2010
Mar. 31, 2010
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Summary of revenues from related party                      
Coal royalty revenues                 $ 279,221 $ 221,761 $ 196,621
Processing fees                 13,475 9,604 7,673
Transportation fees                 16,688 14,564 12,517
Minimums recognized as revenue                 9,148 14,199 1,266
Override revenue                 14,523 11,258 9,251
Total revenues 90,681 103,164 96,532 84,852 77,543 80,752 79,588 63,519 377,683 301,401 256,084
Cline Affiliates [Member]
                      
Summary of revenues from related party                      
Coal royalty revenues                 42,474 32,407 23,325
Processing fees                 2,975 1,337 193
Transportation fees                 16,689 14,324 11,495
Minimums recognized as revenue                   12,400  
Override revenue                 2,691 1,904 2,356
Total revenues                 $ 64,829 $ 62,372 $ 37,369