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Significant Accounting Policies - Depreciation (Details) | 12 Months Ended
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Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($)
Depreciation and amortization expense included in operations |  |  |
Depreciation and amortization expense included in operations | $ 352,000 | $ 151,000 | $ 176,000
Depletion of mineral rights | $ 310,000 | $ 514,000 | $ 510,000
Property and equipment to operations over their estimated service lives minimum (Years) | 5 |  |
Property and equipment to operations over their estimated service lives maximum (Years) | 40 |  |
Office and field equipment useful lives range minimum (years) | 5 |  |
Office and field equipment useful lives range maximum (years) | 30 |  |

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