Segment Reporting - Identifiable Assets by Industry Segment (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Assets	$ 330,842	$ 278,870
Operating Segments
Segment Reporting Information [Line Items]
Assets	330,842	278,870	$ 282,704
Operating Segments | Marketing
Segment Reporting Information [Line Items]
Assets	141,402	119,370	134,745
Operating Segments | Transportation
Segment Reporting Information [Line Items]
Assets	58,483	34,112	29,069
Operating Segments | Oil and natural gas
Segment Reporting Information [Line Items]
Assets	0	0	425
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Assets	$ 130,957	$ 125,388	$ 118,465