Intangible Assets (Schedule of Components of Intangible Assets) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 192,530	$ 188,145
Accumulated Amortization	64,583	59,403
Carrying Value	127,947	128,742
Contract intangibles | Marine Transportation
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	800	800
Accumulated Amortization	589	571
Carrying Value	211	229
Contract intangibles | Offshore pipeline transportation
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	158,101	158,101
Accumulated Amortization	49,233	45,073
Carrying Value	108,868	113,028
Other
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	33,629	29,244
Accumulated Amortization	14,761	13,759
Carrying Value	$ 18,868	$ 15,485