Business Segments - Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information
Total revenue	$ 140,638	$ 187,323	$ 339,521	$ 427,356
Depreciation and Amortization	15,343	15,087	30,582	29,988
Operating Income (Loss) after Eliminations	7,960	5,010	23,560	14,616
Capital Expenditures and Plant Turnaround Costs	6,008	8,627	16,769	18,926
Terminalling and storage
Segment Reporting Information
Total revenue	45,434	52,748	94,842	106,919
Depreciation and Amortization	7,272	7,826	14,728	15,663
Operating Income (Loss) after Eliminations	2,969	4,254	3,727	9,069
Capital Expenditures and Plant Turnaround Costs	3,427	3,668	7,022	9,013
Transportation
Segment Reporting Information
Total revenue	31,485	41,321	70,426	79,116
Depreciation and Amortization	4,328	3,778	8,608	7,348
Operating Income (Loss) after Eliminations	(2,078)	(2,629)	(7,099)	(6,509)
Capital Expenditures and Plant Turnaround Costs	772	746	4,698	3,207
Sulfur services
Segment Reporting Information
Total revenue	33,420	35,856	61,743	67,449
Depreciation and Amortization	3,131	2,854	6,025	5,722
Operating Income (Loss) after Eliminations	8,814	7,528	22,672	13,908
Capital Expenditures and Plant Turnaround Costs	1,739	3,273	4,874	5,479
Natural gas liquids
Segment Reporting Information
Total revenue	30,299	57,398	112,510	173,872
Depreciation and Amortization	612	629	1,221	1,255
Operating Income (Loss) after Eliminations	2,616	456	12,993	7,314
Capital Expenditures and Plant Turnaround Costs	70	940	175	1,227
Operating Revenues
Segment Reporting Information
Total revenue	145,955	194,838	352,814	443,983
Operating Revenues | Terminalling and storage
Segment Reporting Information
Total revenue	46,976	54,351	98,110	110,243
Operating Revenues | Transportation
Segment Reporting Information
Total revenue	35,259	47,233	80,433	92,419
Operating Revenues | Sulfur services
Segment Reporting Information
Total revenue	33,420	35,856	61,756	67,449
Operating Revenues | Natural gas liquids
Segment Reporting Information
Total revenue	30,300	57,398	112,515	173,872
Intersegment Revenues Eliminations
Segment Reporting Information
Total revenue	(5,317)	(7,515)	(13,293)	(16,627)
Intersegment Revenues Eliminations | Terminalling and storage
Segment Reporting Information
Total revenue	(1,542)	(1,603)	(3,268)	(3,324)
Intersegment Revenues Eliminations | Transportation
Segment Reporting Information
Total revenue	(3,774)	(5,912)	(10,007)	(13,303)
Intersegment Revenues Eliminations | Sulfur services
Segment Reporting Information
Total revenue	0	0	(13)	0
Intersegment Revenues Eliminations | Natural gas liquids
Segment Reporting Information
Total revenue	(1)	0	(5)	0
Indirect selling, general and administrative
Segment Reporting Information
Depreciation and Amortization	0	0	0	0
Operating Income (Loss) after Eliminations	(4,361)	(4,599)	(8,733)	(9,166)
Capital Expenditures and Plant Turnaround Costs	$ 0	$ 0	$ 0	$ 0