Intangible Assets (Schedule of Components of Intangible Assets) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 194,302	$ 188,145
Accumulated Amortization	67,154	59,403
Carrying Value	127,148	128,742
Contract intangibles | Marine Transportation
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	800	800
Accumulated Amortization	599	571
Carrying Value	201	229
Contract intangibles | Offshore pipeline transportation
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	158,101	158,101
Accumulated Amortization	51,313	45,073
Carrying Value	106,788	113,028
Other
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	35,401	29,244
Accumulated Amortization	15,242	13,759
Carrying Value	$ 20,159	$ 15,485