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Summary of Significant Accounting Policies - Additional Information (Detail)
shares in Thousands
 3 Months Ended 9 Months Ended
Jun. 01, 2018
Sep. 30, 2018
USD ($)
shares
Dec. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
shares
Sep. 30, 2018
USD ($)
manufactured_units
segment
shares
Sep. 30, 2017
USD ($)
shares
Disaggregation of Revenue [Line Items]            
Revenue   $ 14,796,000   $ 13,925,000 $ 37,929,000 $ 31,547,000
Rail car revenue recognized   2,234,000   2,021,000 6,126,000 5,605,000
Contract liability         0  
Deferred revenue   4,030,000 $ 0   $ 4,030,000  
Deferred revenue balance     0      
Accounts receivables due period         30 days  
Accounts receivables, allowance for doubtful accounts   0 0   $ 0  
Revenue generated from transportation   23,128,000   15,390,000 53,611,000 33,544,000
Inventory write-down   0   0 0 0
Debt fee   728,000 1,372,000   728,000  
Amortization of deferred financing cost   85,000   116,000 223,000 339,000
Accretion of debt discount costs   65,000   0 181,000 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   1,022   451 646 252
Take-or-pay Contracts            
Disaggregation of Revenue [Line Items]            
Revenue   $ 1,426,000   $ 1,169,000 $ 2,094,000 $ 1,244,000
Transferred at Point in Time | Product Concentration Risk | Revenue            
Disaggregation of Revenue [Line Items]            
Concentration risk, percentage   70.00%   56.00% 71.00% 59.00%
Transferred at Point in Time | Rail Car Rental And Transportation | Product Concentration Risk | Revenue            
Disaggregation of Revenue [Line Items]            
Concentration risk, percentage   27.00%   41.00% 27.00% 39.00%
Quickthree Solutions            
Disaggregation of Revenue [Line Items]            
Adjustment period 3 years       3 years  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01            
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]            
Expected percent of revenue recognized   14.00%     14.00%  
Revenue remaining revenue performance obligation, expected timing of satisfaction, period         3 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   86.00%     86.00%  
Revenue remaining revenue performance obligation, expected timing of satisfaction, period         3 years  
Minimum | Quickthree Solutions            
Disaggregation of Revenue [Line Items]            
Measurement basis, manufactured units | manufactured_units         9  
Maximum | Quickthree Solutions            
Disaggregation of Revenue [Line Items]            
Measurement basis, manufactured units | manufactured_units         63