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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