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Hourglass Sands (Narrative) (Details) - USD ($) $ in Thousands | 1 Months Ended | 12 Months Ended
--- | --- | ---
Feb. 28, 2018 | Dec. 31, 2018 | Dec. 31, 2017
Business Acquisition [Line Items] |  |  |
Operating costs and expenses |  | $ 215,363 | $ 190,150
Cash and cash equivalents |  | $ 15,502 | $ 12,483
Hourglass [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Percentage of profit after capital investment and interest is returned |  | 90.00% |
Capitalization |  | $ 8,000 |
Hourglass [Member] | Director [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Business acquisition purchase price | $ 4,000 |  |
Capital Unit, Class A [Member] | Hourglass [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Percentage of profit until capital investment and interest is returned |  | 100.00% |
Hourglass Sands LLC [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Date of acquisition agreement |  | Feb. 01, 2018 |
Business acquisition purchase price | $ 4,000 |  |
Cash and cash equivalents |  | $ 2,900 |
Sand inventory |  | $ 1,300 |
Hourglass Sands LLC [Member] | Director [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Date of acquisition agreement |  | Feb. 01, 2018 |
Hourglass Sands LLC [Member] | Capital Unit, Class A [Member] | Hourglass [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Equity method investment ownership percentage |  | 100.00% |

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