Segment Reporting - Identifiable Assets by Industry Segment (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Segment Reconciliation [Abstract]
Total assets	$ 365,006	$ 296,187
Operating segments
Segment Reconciliation [Abstract]
Total assets	365,006	296,187
Operating segments | Crude Oil Marketing
Segment Reconciliation [Abstract]
Total assets	165,072	128,441
Operating segments | Transportation
Segment Reconciliation [Abstract]
Total assets	68,110	72,247
Operating segments | Pipeline and storage
Segment Reconciliation [Abstract]
Total assets	24,253	24,541
Cash and other
Segment Reconciliation [Abstract]
Total assets	$ 107,571	$ 70,958