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Goodwill and Intangible Assets - Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Goodwill [Line Items]      
Amortization expense $ 3.3 $ 4.5 $ 4.4
Supplier relationships | Texadian      
Goodwill [Line Items]      
Goodwill, impairment loss     7.0
Impairment of intangible assets     $ 2.6
Mid Pac Petroleum, LLC      
Goodwill [Line Items]      
Average useful life 13 years 7 months