Segment Reporting - Identifiable Assets by Industry Segment (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Segment Reconciliation [Abstract]
Total assets	$ 370,425	$ 296,187
Operating segments
Segment Reconciliation [Abstract]
Total assets	370,425	296,187
Operating segments | Crude Oil Marketing
Segment Reconciliation [Abstract]
Total assets	163,053	128,441
Operating segments | Transportation
Segment Reconciliation [Abstract]
Total assets	67,574	72,247
Operating segments | Pipeline and storage
Segment Reconciliation [Abstract]
Total assets	25,662	24,541
Cash and other
Segment Reconciliation [Abstract]
Total assets	$ 114,136	$ 70,958