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Summary of Significant Accounting Policies - Additional Information (Detail) shares in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 USD ($) shares | Dec. 31, 2017 USD ($) | Jun. 30, 2017 USD ($) shares | Jun. 30, 2018 USD ($) segment shares | Jun. 30, 2017 USD ($) shares
Disaggregation of Revenue [Line Items] |  |  |  |  |
Contract liability |  |  |  | $ 0 |
Rail car revenue recognized | $ 2,073,000 |  | $ 1,900,000 | 3,892,000 | $ 3,583,000
Revenue generated from transportation | 11,240,000 |  | 11,018,000 | 23,133,000 | 17,622,000
Deferred revenue balance | 4,805,000 | $ 0 |  | $ 4,805,000 |
Accounts receivables due period |  |  |  | 30 days |
Accounts receivables, allowance for doubtful accounts | 0 | 0 |  | $ 0 |
Cost of goods sold generated from shipping | 16,168,000 |  | 10,852,000 | 30,483,000 | 18,154,000
Inventory write-down | 0 |  | 0 | 0 | 0
Debt fee | 698,000 | 1,372,000 |  | 698,000 |
Amortization of deferred financing cost | 78,000 |  | 117,000 | 138,000 | 222,000
Accretion of debt discount costs | 60,000 |  | 0 | 116,000 | 0
Provisional benefit from Tax Reform Act |  | 8,500,000 |  |  |
Recognized income tax interest and penalties | 0 |  | $ 0 | 0 | $ 0
Probable environmental matters | $ 0 | $ 0 |  | $ 0 |
Number of operating segments | segment |  |  |  | 1 |
Antidilutive securities excluded from computation of earnings per share (in shares) | shares | 670 |  | 10 | 670 | 246
Take-or-pay Contracts |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Revenue recognized for shortfall payments relating to minimum commitments | $ 668,000 |  | $ 74,000 | $ 668,000 | $ 74,000
Transferred at Point in Time | Product Concentration Risk | Revenue |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Concentration risk, percentage | 74.00% |  | 57.00% | 71.00% | 35.00%
Transferred at Point in Time | Rail Car Rental And Transportation | Product Concentration Risk | Revenue |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Concentration risk, percentage | 24.00% |  | 43.00% | 28.00% | 39.00%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-07-01 |  |  |  |  |
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] |  |  |  |  |
Expected percent of revenue recognized | 24.00% |  |  | 24.00% |
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period |  |  |  | 6 months |
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01 |  |  |  |  |
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] |  |  |  |  |
Expected percent of revenue recognized | 76.00% |  |  | 76.00% |
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period |  |  |  | 3 years |

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