Segment Reporting - Identifiable Assets by Industry Segment (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Segment Reconciliation [Abstract]
Total assets	$ 303,127	$ 282,704
Reporting Segments
Segment Reconciliation [Abstract]
Total assets	303,127	282,704
Reporting Segments | Marketing
Segment Reconciliation [Abstract]
Total assets	139,687	134,745
Reporting Segments | Transportation
Segment Reconciliation [Abstract]
Total assets	28,803	29,069
Reporting Segments | Oil and Gas
Segment Reconciliation [Abstract]
Total assets	425	425
Cash and other assets
Segment Reconciliation [Abstract]
Total assets	$ 134,212	$ 118,465