Business Segment Information (Schedule of Total Assets by Reportable Segment) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total consolidated assets	$ 6,935,168	$ 7,137,481
Other assets
Segment Reporting Information [Line Items]
Total consolidated assets	40,967	49,737
Offshore pipeline transportation
Segment Reporting Information [Line Items]
Total consolidated assets	2,417,602	2,486,803
Sodium Minerals & Sulfur Services
Segment Reporting Information [Line Items]
Total consolidated assets	1,839,107	1,848,188
Onshore Facilities & Transportation
Segment Reporting Information [Line Items]
Total consolidated assets	1,824,199	1,927,976
Marine transportation
Segment Reporting Information [Line Items]
Total consolidated assets	$ 813,293	$ 824,777