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Discontinued Operations (Details)
$ in Thousands
3 Months Ended
Dec. 31, 2016
USD ($)
item
Dec. 31, 2015
USD ($)
Sep. 30, 2016
USD ($)
Sep. 30, 2013
USD ($)
Discontinued operations        
Gain (loss) from operations of discontinued operations before income taxes $ 243 $ (346)    
Income tax benefit (provision) (90) 130    
Income (loss) from discontinued operations, net of income taxes $ 153 $ (216)    
Coal Cleaning Business | Discontinued Operations, Disposed of by Sale        
Discontinued operations        
Remaining assets held for sale       $ 0
Number of sales transactions in which the buyer agreed to assume the lease and certain reclamation obligations | item 1      
Number of reclamation obligations | item 1      
Accrued liability for reclamation obligations $ 9,000   $ 9,300