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Mineral Rights - Mineral Rights (Detail) - USD ($) $ in Thousands | Dec. 31, 2015 | Dec. 31, 2014
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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

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- Definition Carrying amount at the balance sheet date of mineral properties, gross of adjustments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 360 -Section 25 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=56951519&loc=d3e64895-109465 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=51678428&loc=d3e61797-109447 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 + Details
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