Business Segment Information (Schedule of Total Assets by Reportable Segment) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Total consolidated assets	$ 6,554,943	$ 6,479,071
Other assets
Segment Reporting Information [Line Items]
Total consolidated assets	43,036	43,060
Offshore pipeline transportation | Operating Segments
Segment Reporting Information [Line Items]
Total consolidated assets	2,336,898	2,359,013
Sodium Minerals & Sulfur Services | Operating Segments
Segment Reporting Information [Line Items]
Total consolidated assets	1,947,472	1,844,845
Onshore Facilities & Transportation | Operating Segments
Segment Reporting Information [Line Items]
Total consolidated assets	1,446,972	1,431,910
Marine transportation | Operating Segments
Segment Reporting Information [Line Items]
Total consolidated assets	$ 780,565	$ 800,243