REPORTABLE SEGMENTS (Details) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2021 USD ($) segment	Jun. 30, 2020 USD ($) segment
Segment results
Number of reportable segments | segment					2	2
Total Operating Revenue	$ 874.9		$ 524.1		$ 1,586.7	$ 1,038.0
Total Operating Income	213.9		51.2		334.1	64.2
Interest expense, net	(5.5)		(8.2)		(12.8)	(16.8)
Other income (expense), net	1.5		1.5		2.9	2.1
Income before Income Taxes	209.9		44.5		324.2	49.5
Income taxes	(47.4)		(11.7)		(74.5)	(12.9)
Net Income	162.5	$ 87.2	32.8	$ 3.8	249.7	36.6
Income from SSAT	12.8		3.7		22.0	7.7
SSAT
Segment results
Income from SSAT	12.8		3.7		22.0	7.7
Intersegment Eliminations
Segment results
Total Operating Revenue	49.0		24.2		82.7	43.6
Ocean Transportation
Segment results
Total Operating Revenue	682.9		410.8		1,243.4	811.7
Total Operating Income	201.0		42.3		315.1	50.2
Income from SSAT	12.8		3.7
Ocean Transportation | Intersegment Eliminations
Segment results
Total Operating Revenue	18.7		11.9		33.8	21.3
Logistics
Segment results
Total Operating Revenue	192.0		113.3		343.3	226.3
Total Operating Income	12.9		8.9		19.0	14.0
Logistics | Intersegment Eliminations
Segment results
Total Operating Revenue	$ 30.3		$ 12.3		$ 48.9	$ 22.3