Intangible Assets (Schedule of Components of Intangible Assets) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 189,850	$ 188,145
Accumulated Amortization	62,002	59,403
Carrying Value	127,848	128,742
Contract intangibles | Marine Transportation
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	800	800
Accumulated Amortization	580	571
Carrying Value	220	229
Contract intangibles | Offshore pipeline transportation
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	158,101	158,101
Accumulated Amortization	47,153	45,073
Carrying Value	110,948	113,028
Other
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	30,949	29,244
Accumulated Amortization	14,269	13,759
Carrying Value	$ 16,680	$ 15,485