Segment Reporting - Identifiable Assets by Industry Segment (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Segment Reconciliation [Abstract]
Total assets	$ 332,302	$ 296,187
Operating segments
Segment Reconciliation [Abstract]
Total assets	332,302	296,187
Operating segments | Marketing
Segment Reconciliation [Abstract]
Total assets	153,723	128,441
Operating segments | Transportation
Segment Reconciliation [Abstract]
Total assets	65,572	72,247
Operating segments | Pipeline and storage
Segment Reconciliation [Abstract]
Total assets	24,290	24,541
Cash and other
Segment Reconciliation [Abstract]
Total assets	$ 88,717	$ 70,958