Business Segment Information (Schedule of Total Assets by Reportable Segment) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total consolidated assets	$ 7,057,632	$ 7,137,481
Other assets
Segment Reporting Information [Line Items]
Total consolidated assets	47,348	49,737
Offshore pipeline transportation
Segment Reporting Information [Line Items]
Total consolidated assets	2,451,743	2,486,803
Sodium Minerals & Sulfur Services
Segment Reporting Information [Line Items]
Total consolidated assets	1,850,695	1,848,188
Marine transportation
Segment Reporting Information [Line Items]
Total consolidated assets	815,858	824,777
Onshore Facilities & Transportation
Segment Reporting Information [Line Items]
Total consolidated assets	$ 1,891,988	$ 1,927,976