Accounting Policies and Basis of Presentation - Narrative (Details) $ in Thousands	Mar. 31, 2018 USD ($) rail_car office_space	Dec. 31, 2017 USD ($)
Accounting Policies [Abstract]
Debt instrument, fair value disclosure	$ 32,683	$ 24,838
Long-term debt	$ 32,235	$ 24,554
Property under operating lease \| Rail cars
Property Subject to or Available for Operating Lease [Line Items]
Property subject to operating leases, number of units \| rail_car	251
Property under operating lease \| Office space
Property Subject to or Available for Operating Lease [Line Items]
Property subject to operating leases, number of units \| office_space	1

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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The number of units (items of property) under operating lease arrangements.

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