| Segment Reporting [Table Text Block] |
| (In Thousands) | | Three Months Ended | | | Six Months Ended | |
| | | June 30, 2026 | | | June 30, 2025 | | | June 30, 2026 | | | June 30, 2025 | |
| Americas: | | | | | | | | | | | | | | | | |
| Vessel revenues | | $ | 53,969 | | | $ | 68,758 | | | $ | 112,495 | | | $ | 123,610 | |
| | | | | | | | | | | | | | | | | |
| Vessel operating costs: | | | | | | | | | | | | | | | | |
| Crew costs | | | 17,972 | | | | 19,652 | | | | 37,701 | | | | 37,092 | |
| Repair and maintenance | | | 5,706 | | | | 4,830 | | | | 10,934 | | | | 9,096 | |
| Insurance | | | 429 | | | | 351 | | | | 891 | | | | 922 | |
| Fuel, lube and supplies | | | 3,252 | | | | 2,215 | | | | 6,139 | | | | 4,832 | |
| Other | | | 5,276 | | | | 5,965 | | | | 10,840 | | | | 16,094 | |
| Total vessel operating costs | | | 32,635 | | | | 33,013 | | | | 66,505 | | | | 68,036 | |
| General and administrative expense | | | 4,143 | | | | 3,785 | | | | 7,894 | | | | 7,327 | |
| Depreciation and amortization | | | 9,806 | | | | 11,718 | | | | 21,144 | | | | 23,110 | |
| Vessel operating profit | | | 7,385 | | | | 20,242 | | | | 16,952 | | | | 25,137 | |
| | | | | | | | | | | | | | | | | |
| Additions to properties and equipment | | $ | 1,268 | | | $ | 353 | | | $ | 2,734 | | | $ | 2,286 | |
| | | | | | | | | | | | | | | | | |
| Total assets | | $ | 309,457 | | | $ | 397,790 | | | $ | 309,457 | | | $ | 397,790 | |
| (In Thousands) | | Three Months Ended | | | Six Months Ended | |
| | | June 30, 2026 | | | June 30, 2025 | | | June 30, 2026 | | | June 30, 2025 | |
| Asia Pacific: | | | | | | | | | | | | | | | | |
| Vessel revenues | | $ | 47,948 | | | $ | 45,696 | | | $ | 94,512 | | | $ | 93,924 | |
| | | | | | | | | | | | | | | | | |
| Vessel operating costs: | | | | | | | | | | | | | | | | |
| Crew costs | | | 17,630 | | | | 18,518 | | | | 33,497 | | | | 38,849 | |
| Repair and maintenance | | | 3,136 | | | | 3,365 | | | | 6,093 | | | | 5,635 | |
| Insurance | | | 283 | | | | 176 | | | | 537 | | | | 500 | |
| Fuel, lube and supplies | | | 3,048 | | | | 1,789 | | | | 5,561 | | | | 3,556 | |
| Other | | | 2,095 | | | | 2,317 | | | | 3,849 | | | | 4,435 | |
| Total vessel operating costs | | | 26,192 | | | | 26,165 | | | | 49,537 | | | | 52,975 | |
| General and administrative expense | | | 2,442 | | | | 2,050 | | | | 4,667 | | | | 4,470 | |
| Depreciation and amortization | | | 6,207 | | | | 5,703 | | | | 11,731 | | | | 11,021 | |
| Vessel operating profit | | | 13,107 | | | | 11,778 | | | | 28,577 | | | | 25,458 | |
| | | | | | | | | | | | | | | | | |
| Additions to properties and equipment | | $ | (102 | ) | | $ | 375 | | | $ | 444 | | | $ | 967 | |
| | | | | | | | | | | | | | | | | |
| Total assets | | $ | 167,986 | | | $ | 153,018 | | | $ | 167,986 | | | $ | 153,018 | |
| (In Thousands) | | Three Months Ended | | | Six Months Ended | |
| | | June 30, 2026 | | | June 30, 2025 | | | June 30, 2026 | | | June 30, 2025 | |
| Middle East: | | | | | | | | | | | | | | | | |
| Vessel revenues | | $ | 48,483 | | | $ | 40,215 | | | $ | 94,052 | | | $ | 83,517 | |
| | | | | | | | | | | | | | | | | |
| Vessel operating costs: | | | | | | | | | | | | | | | | |
| Crew costs | | | 17,314 | | | | 13,302 | | | | 32,841 | | | | 26,582 | |
| Repair and maintenance | | | 4,514 | | | | 4,261 | | | | 8,370 | | | | 8,361 | |
| Insurance | | | 852 | | | | 343 | | | | 1,615 | | | | 872 | |
| Fuel, lube and supplies | | | 3,548 | | | | 3,250 | | | | 6,232 | | | | 5,289 | |
| Other | | | 4,044 | | | | 4,661 | | | | 8,119 | | | | 9,249 | |
| Total vessel operating costs | | | 30,272 | | | | 25,817 | | | | 57,177 | | | | 50,353 | |
| General and administrative expense | | | 2,316 | | | | 2,847 | | | | 4,593 | | | | 5,784 | |
| Depreciation and amortization | | | 9,711 | | | | 7,657 | | | | 19,470 | | | | 14,923 | |
| Vessel operating profit | | | 6,184 | | | | 3,894 | | | | 12,812 | | | | 12,457 | |
| | | | | | | | | | | | | | | | | |
| Additions to properties and equipment | | $ | 2,596 | | | $ | 329 | | | $ | 6,939 | | | $ | 1,183 | |
| | | | | | | | | | | | | | | | | |
| Total assets | | $ | 208,277 | | | $ | 180,769 | | | $ | 208,277 | | | $ | 180,769 | |
| (In Thousands) | | Three Months Ended | | | Six Months Ended | |
| | | June 30, 2026 | | | June 30, 2025 | | | June 30, 2026 | | | June 30, 2025 | |
| Europe Mediterranean: | | | | | | | | | | | | | | | | |
| Vessel revenues | | $ | 112,076 | | | $ | 99,280 | | | $ | 199,021 | | | $ | 177,485 | |
| | | | | | | | | | | | | | | | | |
| Vessel operating costs: | | | | | | | | | | | | | | | | |
| Crew costs | | | 34,312 | | | | 29,342 | | | | 66,011 | | | | 56,453 | |
| Repair and maintenance | | | 9,419 | | | | 5,736 | | | | 16,985 | | | | 12,447 | |
| Insurance | | | 810 | | | | 417 | | | | 1,509 | | | | 1,265 | |
| Fuel, lube and supplies | | | 5,229 | | | | 2,153 | | | | 9,880 | | | | 5,300 | |
| Other | | | 5,981 | | | | 6,187 | | | | 11,747 | | | | 10,925 | |
| Total vessel operating costs | | | 55,751 | | | | 43,835 | | | | 106,132 | | | | 86,390 | |
| General and administrative expense | | | 3,401 | | | | 3,385 | | | | 7,163 | | | | 7,048 | |
| Depreciation and amortization | | | 26,075 | | | | 22,833 | | | | 50,832 | | | | 47,442 | |
| Vessel operating profit | | | 26,849 | | | | 29,227 | | | | 34,894 | | | | 36,605 | |
| | | | | | | | | | | | | | | | | |
| Additions to properties and equipment | | $ | 8,454 | | | $ | 584 | | | $ | 13,666 | | | $ | 1,898 | |
| | | | | | | | | | | | | | | | | |
| Total assets | | $ | 652,900 | | | $ | 628,740 | | | $ | 652,900 | | | $ | 628,740 | |
| (In Thousands) | | Three Months Ended | | | Six Months Ended | |
| | | June 30, 2026 | | | June 30, 2025 | | | June 30, 2026 | | | June 30, 2025 | |
| West Africa: | | | | | | | | | | | | | | | | |
| Vessel revenues | | $ | 77,255 | | | $ | 82,909 | | | $ | 163,069 | | | $ | 189,021 | |
| | | | | | | | | | | | | | | | | |
| Vessel operating costs: | | | | | | | | | | | | | | | | |
| Crew costs | | | 17,997 | | | | 18,662 | | | | 34,062 | | | | 37,613 | |
| Repair and maintenance | | | 6,178 | | | | 5,745 | | | | 11,375 | | | | 10,352 | |
| Insurance | | | 565 | | | | 353 | | | | 1,047 | | | | 1,115 | |
| Fuel, lube and supplies | | | 5,969 | | | | 5,700 | | | | 9,688 | | | | 10,508 | |
| Other | | | 5,187 | | | | 8,064 | | | | 11,410 | | | | 14,991 | |
| Total vessel operating costs | | | 35,896 | | | | 38,524 | | | | 67,582 | | | | 74,579 | |
| General and administrative expense | | | 3,254 | | | | 2,888 | | | | 6,225 | | | | 5,434 | |
| Depreciation and amortization | | | 13,493 | | | | 15,480 | | | | 27,420 | | | | 31,378 | |
| Vessel operating profit | | | 24,612 | | | | 26,017 | | | | 61,842 | | | | 77,630 | |
| | | | | | | | | | | | | | | | | |
| Additions to properties and equipment | | $ | 1,488 | | | $ | 3,502 | | | $ | 3,225 | | | $ | 16,334 | |
| | | | | | | | | | | | | | | | | |
| Total assets | | $ | 388,255 | | | $ | 499,787 | | | $ | 388,255 | | | $ | 499,787 | |
| (In Thousands) | | Three Months Ended | | | Six Months Ended | |
| | | June 30, 2026 | | | June 30, 2025 | | | June 30, 2026 | | | June 30, 2025 | |
| World Wide: | | | | | | | | | | | | | | | | |
| Revenues: | | | | | | | | | | | | | | | | |
| Vessel revenues | | $ | 339,731 | | | $ | 336,858 | | | $ | 663,149 | | | $ | 667,557 | |
| Other operating revenues | | | 2,556 | | | | 4,573 | | | | 5,360 | | | | 7,318 | |
| Total revenue | | | 342,287 | | | | 341,431 | | | | 668,509 | | | | 674,875 | |
| | | | | | | | | | | | | | | | | |
| Vessel operating costs: | | | | | | | | | | | | | | | | |
| Crew costs | | | 105,225 | | | | 99,476 | | | | 204,112 | | | | 196,589 | |
| Repair and maintenance | | | 28,953 | | | | 23,937 | | | | 53,757 | | | | 45,891 | |
| Insurance | | | 2,939 | | | | 1,640 | | | | 5,599 | | | | 4,674 | |
| Fuel, lube and supplies | | | 21,046 | | | | 15,107 | | | | 37,500 | | | | 29,485 | |
| Other | | | 22,583 | | | | 27,194 | | | | 45,965 | | | | 55,694 | |
| Total vessel operating costs | | | 180,746 | | | | 167,354 | | | | 346,933 | | | | 332,333 | |
| Costs of other operating revenues | | | 1,072 | | | | 3,108 | | | | 1,833 | | | | 4,538 | |
| General and administrative expense | | | 15,556 | | | | 14,955 | | | | 30,542 | | | | 30,063 | |
| Depreciation and amortization | | | 65,292 | | | | 63,391 | | | | 130,597 | | | | 127,874 | |
| Operating profit | | | 79,621 | | | | 92,623 | | | | 158,604 | | | | 180,067 | |
| Corporate expenses | | | (20,409 | ) | | | (17,181 | ) | | | (40,296 | ) | | | (32,116 | ) |
| Gain on asset dispositions, net | | | 3,316 | | | | 5,480 | | | | 3,204 | | | | 8,018 | |
| Operating income | | | 62,528 | | | | 80,922 | | | | 121,512 | | | | 155,969 | |
| | | | | | | | | | | | | | | | | |
| Segment additions to properties and equipment | | $ | 13,704 | | | $ | 5,143 | | | $ | 27,008 | | | $ | 22,668 | |
| Corporate additions to properties and equipment | | | 1,219 | | | | 1,712 | | | | 2,800 | | | | 3,551 | |
| Total additions to properties and equipment | | $ | 14,923 | | | $ | 6,855 | | | $ | 29,808 | | | $ | 26,219 | |
| | | | | | | | | | | | | | | | | |
| Segment assets | | $ | 1,726,875 | | | $ | 1,860,104 | | | $ | 1,726,875 | | | $ | 1,860,104 | |
| Corporate assets | | | 639,385 | | | | 214,268 | | | | 639,385 | | | | 214,268 | |
| Total assets | | $ | 2,366,260 | | | $ | 2,074,372 | | | $ | 2,366,260 | | | $ | 2,074,372 | |
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