Segment Reporting - Identifiable assets by industry segment (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Assets	$ 278,870	$ 282,704
Operating Segments
Segment Reporting Information [Line Items]
Assets	278,870	282,704	$ 246,872
Operating Segments | Marketing
Segment Reporting Information [Line Items]
Assets	119,370	134,745	107,257
Operating Segments | Transportation
Segment Reporting Information [Line Items]
Assets	34,112	29,069	32,120
Operating Segments | Oil and natural gas
Segment Reporting Information [Line Items]
Assets	0	425	7,279
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Assets	$ 125,388	$ 118,465	$ 100,216