Segment Information - Operating Results, Capital Expenditures And Assets By Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Operating Revenues	$ 175,414	$ 200,658	$ 540,295	$ 607,205
Costs and Expenses:
Operating	128,546	147,386	400,752	437,368
Administrative and general	24,560	24,923	76,785	78,383
Depreciation and amortization	17,306	16,975	52,620	51,120
Total costs and expenses	170,412	189,284	530,157	566,871
Gains (Losses) on Asset Dispositions, Net	618	1,145	9,200	2,259
Operating Income (Loss)	5,620	12,519	19,338	42,593
Other Income (Expense):
Foreign currency gains (losses), net	(203)	(1,877)	(3,042)	(1,663)
Other, net	2,242	505	2,992	(114)
Equity in Earnings (Losses) of 50% or Less Owned Companies, Net of Tax	(1,102)	(618)	(8,877)	(3,448)
Income Before Income Tax Expense and Equity in Earnings (Losses) of 50% or Less Owned Companies	5,843	7,896	12,026	46,390
Capital Expenditures			16,326	17,351
Historical cost	1,442,442	1,424,907	1,442,442	1,424,907	$ 1,442,382
Accumulated depreciation	(663,277)	(607,727)	(663,277)	(607,727)	(624,024)
Net property and equipment	779,165	817,180	779,165	817,180	818,358
Operating Lease Right-of-Use Assets	124,855	153,464	124,855	153,464	144,539
Investments, at Equity, and Advances to 50% or Less Owned Companies	152,744	154,968	152,744	154,968	157,108
Inventories	3,871	5,224	3,871	5,224	5,255
Goodwill	32,616	32,668	32,616	32,668	32,701
Intangible Assets	21,041	21,884	21,041	21,884	20,996
Other current and long-term assets, excluding cash and near cash assets	278,706	256,876	278,706	256,876
Total Assets	1,499,729	1,530,246	1,499,729	1,530,246	$ 1,512,972
Ocean Services
Segment Reporting Information [Line Items]
Operating Revenues	88,771	102,661	280,997	321,614
Inland Services
Segment Reporting Information [Line Items]
Operating Revenues	64,069	72,020	188,893	199,077
Witt O'Brien's LLC
Segment Reporting Information [Line Items]
Operating Revenues	20,505	24,342	64,139	80,932
Other Segment
Segment Reporting Information [Line Items]
Operating Revenues	2,069	1,635	6,266	5,582
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating Revenues	(13)	(3)	(58)	(109)
Intersegment Eliminations | Ocean Services
Segment Reporting Information [Line Items]
Operating Revenues	0	0	0	0
Intersegment Eliminations | Inland Services
Segment Reporting Information [Line Items]
Operating Revenues	0	0	0	0
Intersegment Eliminations | Witt O'Brien's LLC
Segment Reporting Information [Line Items]
Operating Revenues	(13)	(3)	(58)	(109)
Intersegment Eliminations | Other Segment
Segment Reporting Information [Line Items]
Operating Revenues	0	0	0	0
Operating Segments | Ocean Services
Segment Reporting Information [Line Items]
Operating Revenues	88,771	102,661	280,997	321,614
Costs and Expenses:
Operating	59,985	66,888	195,416	208,050
Administrative and general	10,436	9,404	29,960	29,025
Depreciation and amortization	10,124	10,191	30,676	30,758
Total costs and expenses	80,545	86,483	256,052	267,833
Gains (Losses) on Asset Dispositions, Net	191	804	313	1,170
Operating Income (Loss)	8,417	16,982	25,258	54,951
Other Income (Expense):
Foreign currency gains (losses), net	213	(104)	218	(150)
Other, net	5	505	9	(118)
Equity in Earnings (Losses) of 50% or Less Owned Companies, Net of Tax	505	(242)	(2,278)	569
Segment Profit (Loss)	9,140	17,141	23,207	55,252
Capital Expenditures			6,659	866
Historical cost	939,141	928,321	939,141	928,321
Accumulated depreciation	(410,195)	(370,679)	(410,195)	(370,679)
Net property and equipment	528,946	557,642	528,946	557,642
Operating Lease Right-of-Use Assets	94,057	115,840	94,057	115,840
Investments, at Equity, and Advances to 50% or Less Owned Companies	78,749	77,203	78,749	77,203
Inventories	1,334	2,056	1,334	2,056
Goodwill	1,852	1,852	1,852	1,852
Intangible Assets	6,641	7,969	6,641	7,969
Other current and long-term assets, excluding cash and near cash assets	44,628	65,069	44,628	65,069
Total Assets	756,207	827,631	756,207	827,631
Operating Segments | Inland Services
Segment Reporting Information [Line Items]
Operating Revenues	64,069	72,020	188,893	199,077
Costs and Expenses:
Operating	54,338	62,775	159,172	171,506
Administrative and general	3,321	3,327	10,101	9,816
Depreciation and amortization	6,036	5,694	18,264	17,118
Total costs and expenses	63,695	71,796	187,537	198,440
Gains (Losses) on Asset Dispositions, Net	427	330	8,827	1,080
Operating Income (Loss)	801	554	10,183	1,717
Other Income (Expense):
Foreign currency gains (losses), net	(439)	(1,729)	(3,264)	(1,461)
Other, net	1,939	0	1,936	0
Equity in Earnings (Losses) of 50% or Less Owned Companies, Net of Tax	(1,141)	(1,084)	(5,218)	(4,174)
Segment Profit (Loss)	1,160	(2,259)	3,637	(3,918)
Capital Expenditures			5,940	15,408
Historical cost	457,745	457,263	457,745	457,263
Accumulated depreciation	(223,205)	(210,629)	(223,205)	(210,629)
Net property and equipment	234,540	246,634	234,540	246,634
Operating Lease Right-of-Use Assets	26,003	33,059	26,003	33,059
Investments, at Equity, and Advances to 50% or Less Owned Companies	51,577	55,777	51,577	55,777
Inventories	2,108	2,729	2,108	2,729
Goodwill	2,258	2,310	2,258	2,310
Intangible Assets	6,376	7,870	6,376	7,870
Other current and long-term assets, excluding cash and near cash assets	71,343	73,818	71,343	73,818
Total Assets	394,205	422,197	394,205	422,197
Operating Segments | Witt O'Brien's LLC
Segment Reporting Information [Line Items]
Operating Revenues	20,518	24,345	64,197	81,041
Costs and Expenses:
Operating	12,793	16,323	41,588	53,786
Administrative and general	4,913	5,718	17,553	18,951
Depreciation and amortization	359	210	974	625
Total costs and expenses	18,065	22,251	60,115	73,362
Gains (Losses) on Asset Dispositions, Net	0	10	0	10
Operating Income (Loss)	2,453	2,104	4,082	7,689
Other Income (Expense):
Foreign currency gains (losses), net	(18)	0	(15)	0
Other, net	0	(1)	70	(6)
Equity in Earnings (Losses) of 50% or Less Owned Companies, Net of Tax	130	764	378	569
Segment Profit (Loss)	2,565	2,867	4,515	8,252
Capital Expenditures			0	47
Historical cost	1,145	1,274	1,145	1,274
Accumulated depreciation	(1,072)	(1,106)	(1,072)	(1,106)
Net property and equipment	73	168	73	168
Operating Lease Right-of-Use Assets	1,805	3,442	1,805	3,442
Investments, at Equity, and Advances to 50% or Less Owned Companies	827	904	827	904
Inventories	271	193	271	193
Goodwill	28,506	28,506	28,506	28,506
Intangible Assets	8,024	6,045	8,024	6,045
Other current and long-term assets, excluding cash and near cash assets	122,364	104,206	122,364	104,206
Total Assets	161,870	143,464	161,870	143,464
Operating Segments | Other Segment
Segment Reporting Information [Line Items]
Operating Revenues	2,069	1,635	6,266	5,582
Costs and Expenses:
Operating	1,442	1,404	4,631	4,129
Administrative and general	706	846	2,707	2,522
Depreciation and amortization	459	501	1,693	1,483
Total costs and expenses	2,607	2,751	9,031	8,134
Gains (Losses) on Asset Dispositions, Net	0	34	60	32
Operating Income (Loss)	(538)	(1,082)	(2,705)	(2,520)
Other Income (Expense):
Foreign currency gains (losses), net	0	0	0	0
Other, net	1	0	1	0
Equity in Earnings (Losses) of 50% or Less Owned Companies, Net of Tax	(596)	(56)	(1,759)	(412)
Segment Profit (Loss)	(1,133)	(1,138)	(4,463)	(2,932)
Capital Expenditures			2,201	1,030
Historical cost	11,922	7,928	11,922	7,928
Accumulated depreciation	(4,061)	(1,951)	(4,061)	(1,951)
Net property and equipment	7,861	5,977	7,861	5,977
Operating Lease Right-of-Use Assets	0	0	0	0
Investments, at Equity, and Advances to 50% or Less Owned Companies	21,591	21,084	21,591	21,084
Inventories	158	246	158	246
Goodwill	0	0	0	0
Intangible Assets	0	0	0	0
Other current and long-term assets, excluding cash and near cash assets	3,348	3,377	3,348	3,377
Total Assets	32,958	30,684	32,958	30,684
Corporate And Eliminations
Segment Reporting Information [Line Items]
Operating Revenues			(58)	(109)
Costs and Expenses:
Operating	(12)	(4)	(55)	(103)
Administrative and general	5,184	5,628	16,464	18,069
Depreciation and amortization	328	379	1,013	1,136
Total costs and expenses	5,500	6,003	17,422	19,102
Gains (Losses) on Asset Dispositions, Net	0	(33)	0	(33)
Operating Income (Loss)	(5,513)	(6,039)	(17,480)	(19,244)
Other Income (Expense):
Foreign currency gains (losses), net	41	(44)	19	(52)
Other, net	297	1	976	10
Equity in Earnings (Losses) of 50% or Less Owned Companies, Net of Tax	0	0	0	0
Capital Expenditures			1,526	0
Historical cost	32,489	30,121	32,489	30,121
Accumulated depreciation	(24,744)	(23,362)	(24,744)	(23,362)
Net property and equipment	7,745	6,759	7,745	6,759
Operating Lease Right-of-Use Assets	2,990	1,123	2,990	1,123
Investments, at Equity, and Advances to 50% or Less Owned Companies	0	0	0	0
Inventories	0	0	0	0
Goodwill	0	0	0	0
Intangible Assets	0	0	0	0
Other current and long-term assets, excluding cash and near cash assets	37,023	10,406	37,023	10,406
Segment Reconciling Items
Other Income (Expense):
Other, net	(1,816)	(3,251)	(7,262)	5,574
Equity in Earnings (Losses) of 50% or Less Owned Companies, Net of Tax	1,102	618	8,877	3,448
Cash and near cash assets	$ 106,731	$ 87,982	$ 106,731	$ 87,982