Business Segment Information (Schedule of Total Assets by Reportable Segment) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total consolidated assets	$ 7,109,836	$ 7,137,481
Other assets
Segment Reporting Information [Line Items]
Total consolidated assets	59,328	49,737
Offshore pipeline transportation
Segment Reporting Information [Line Items]
Total consolidated assets	2,472,742	2,486,803
Sodium Minerals & Sulfur Services
Segment Reporting Information [Line Items]
Total consolidated assets	1,842,607	1,848,188
Marine transportation
Segment Reporting Information [Line Items]
Total consolidated assets	819,057	824,777
Onshore Facilities & Transportation
Segment Reporting Information [Line Items]
Total consolidated assets	$ 1,916,102	$ 1,927,976