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Major Lessees - Revenues from Lessees that Exceeded Ten Percent of Total Revenues and Other Income (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Foresight Energy      
Operating Leased Assets [Line Items]      
Revenues $ 86,614 $ 81,546 $ 88,432
Percent 17.70% 20.40% 24.70%
Alpha Natural Resources      
Operating Leased Assets [Line Items]      
Revenues $ 34,364 $ 48,783 $ 55,147
Percent 7.00% 12.20% 15.40%