Segment Reporting - Identifiable Assets by Industry Segment (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Segment Reconciliation [Abstract]
Total assets	$ 314,635	$ 278,870
Reporting Segments
Segment Reconciliation [Abstract]
Total assets	314,635	278,870
Reporting Segments | Marketing
Segment Reconciliation [Abstract]
Total assets	135,755	119,370
Reporting Segments | Transportation
Segment Reconciliation [Abstract]
Total assets	40,796	34,112
Cash and other
Segment Reconciliation [Abstract]
Total assets	$ 138,084	$ 125,388