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Commitments and Contingencies Additional Information (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Commitments and Contingencies Disclosure [Line Items]    
Loss Contingency, Range of Possible Loss, Portion Not Accrued $ 549,758  
Murray Energy [Member]    
Commitments and Contingencies Disclosure [Line Items]    
Contractual Obligation $ 1,285 $ 1,362