REPORTABLE SEGMENTS (Details) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2024 USD ($)	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Sep. 30, 2023 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Sep. 30, 2024 USD ($) segment	Sep. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)
Segment results
Number of segments | segment							2
Total Operating Revenue	$ 962.0			$ 827.5			$ 2,531.5	$ 2,305.7
Total Operating Income	242.3			132.1			403.8	267.5
Interest income	10.4			9.3			38.0	26.2
Interest expense	(1.8)			(2.4)			(6.1)	(9.8)
Other income (expense), net	1.9			1.2			5.5	4.8
Income before Taxes	252.8			140.2			441.2	288.7
Income taxes	(53.7)			(20.3)			(92.8)	(54.0)
Net Income	199.1	$ 113.2	$ 36.1	119.9	$ 80.8	$ 34.0	348.4	234.7
Deferred dry-docking amortization							21.0	18.6
Total Assets	4,442.2						4,442.2		$ 4,294.6
Income (Loss) from SSAT	6.9			1.3			8.5	(1.9)
SSAT
Segment results
Income (Loss) from SSAT	6.9			1.3			8.5	(1.9)
Intersegment Eliminations
Segment results
Total Operating Revenue	72.8			61.1			181.2	155.6
Ocean Transportation
Segment results
Income (Loss) from SSAT	6.9			1.3			8.5	(1.9)
Ocean Transportation | Operating segments
Segment results
Total Operating Revenue	798.7			669.4			2,067.6	1,837.3
Total Operating Income	226.9			118.2			363.5	228.4
Ocean Transportation | Intersegment Eliminations
Segment results
Total Operating Revenue	26.7			22.5			69.2	57.6
Logistics | Operating segments
Segment results
Total Operating Revenue	163.3			158.1			463.9	468.4
Total Operating Income	15.4			13.9			40.3	39.1
Logistics | Intersegment Eliminations
Segment results
Total Operating Revenue	$ 46.1			$ 38.6			$ 112.0	$ 98.0