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Mineral Rights - Mineral Rights (Detail) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]    
Carrying Value $ 1,546,267 $ 2,328,471
Accumulated Depletion (452,240) (546,619)
Net Book Value 1,094,027 1,781,852
Coal, Hard Mineral Royalty and Other    
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]    
Carrying Value 1,278,274 1,680,169
Accumulated Depletion (432,260) (505,582)
Net Book Value 846,014 1,174,587
VantaCore    
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]    
Carrying Value 112,700 87,907
Accumulated Depletion (3,082) (482)
Net Book Value 109,618 87,425
Oil and Gas    
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]    
Carrying Value 155,293 560,395
Accumulated Depletion (16,898) (40,555)
Net Book Value $ 138,395 $ 519,840