REPORTABLE SEGMENTS (Details) $ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($) segment	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)
Segment results
Number of segments | segment	2
Total Operating Revenue	$ 722.1	$ 704.8
Total Operating Income	36.9	38.7
Interest income	8.8	8.2
Interest expense	(2.2)	(4.5)
Other income (expense), net	1.8	1.8
Income before Taxes	45.3	44.2
Income taxes	(9.2)	(10.2)
Net Income	36.1	34.0
Deferred dry-docking amortization	6.8	6.2
Total Assets	4,219.9		$ 4,294.6
Income (Loss) from SSAT	0.4	(1.8)
SSAT
Segment results
Income (Loss) from SSAT	0.4	(1.8)
Intersegment Eliminations
Segment results
Total Operating Revenue	46.6	40.1
Ocean Transportation
Segment results
Income (Loss) from SSAT	0.4	(1.8)
Ocean Transportation | Operating segments
Segment results
Total Operating Revenue	579.0	551.0
Total Operating Income	27.6	27.8
Ocean Transportation | Intersegment Eliminations
Segment results
Total Operating Revenue	19.7	16.1
Logistics | Operating segments
Segment results
Total Operating Revenue	143.1	153.8
Total Operating Income	9.3	10.9
Logistics | Intersegment Eliminations
Segment results
Total Operating Revenue	$ 26.9	$ 24.0