REPORTABLE SEGMENTS (Details) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Jun. 30, 2024 USD ($) segment	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)
Segment results
Number of segments | segment					2
Total Operating Revenue	$ 847.4		$ 773.4		$ 1,569.5	$ 1,478.2
Total Operating Income	124.6		96.7		161.5	135.4
Interest income	18.8		8.7		27.6	16.9
Interest expense	(2.1)		(2.9)		(4.3)	(7.4)
Other income (expense), net	1.8		1.8		3.6	3.6
Income before Taxes	143.1		104.3		188.4	148.5
Income taxes	(29.9)		(23.5)		(39.1)	(33.7)
Net Income	113.2	$ 36.1	80.8	$ 34.0	149.3	114.8
Deferred dry-docking amortization					13.7	12.4
Total Assets	4,273.9				4,273.9		$ 4,294.6
Income (Loss) from SSAT	1.2		(1.4)		1.6	(3.2)
SSAT
Segment results
Income (Loss) from SSAT	1.2		(1.4)		1.6	(3.2)
Intersegment Eliminations
Segment results
Total Operating Revenue	61.8		54.4		108.4	94.5
Ocean Transportation
Segment results
Income (Loss) from SSAT	1.2		(1.4)		1.6	(3.2)
Ocean Transportation | Operating segments
Segment results
Total Operating Revenue	689.9		616.9		1,268.9	1,167.9
Total Operating Income	109.0		82.4		136.6	110.2
Ocean Transportation | Intersegment Eliminations
Segment results
Total Operating Revenue	22.8		19.0		42.5	35.1
Logistics | Operating segments
Segment results
Total Operating Revenue	157.5		156.5		300.6	310.3
Total Operating Income	15.6		14.3		24.9	25.2
Logistics | Intersegment Eliminations
Segment results
Total Operating Revenue	$ 39.0		$ 35.4		$ 65.9	$ 59.4