Segment Reporting - Identifiable Assets by Industry Segment (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Segment Reconciliation [Abstract]
Total assets	$ 301,764	$ 278,870
Reporting Segments
Segment Reconciliation [Abstract]
Total assets	301,764	278,870
Reporting Segments | Marketing
Segment Reconciliation [Abstract]
Total assets	113,554	119,370
Reporting Segments | Transportation
Segment Reconciliation [Abstract]
Total assets	47,688	34,112
Cash and other
Segment Reconciliation [Abstract]
Total assets	$ 140,522	$ 125,388