REPORTABLE SEGMENTS (Details) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($) segment
Segment results
Number of reportable segments | segment					2	2
Total Operating Revenue	$ 524.1		$ 557.9		$ 1,038.0	$ 1,090.3
Total Operating Income	51.2		31.0		64.2	48.5
Interest expense, net	(8.2)		(6.1)		(16.8)	(10.7)
Other income (expense), net	1.5		0.8		2.1	1.4
Income before Income Taxes	44.5		25.7		49.5	39.2
Income tax expense	(11.7)		(7.3)		(12.9)	(8.3)
Net Income	32.8	$ 3.8	18.4	$ 12.5	36.6	30.9
Income from SSAT	3.7		0.9		7.7	9.4
Operating segments
Segment results
Total Operating Revenue	524.1		557.9		1,038.0	1,090.3
Total Operating Income	51.2		31.0		64.2	48.5
Intersegment Eliminations
Segment results
Total Operating Revenue	24.2		25.3		43.6	47.1
Ocean Transportation
Segment results
Total Operating Revenue	410.8		415.4
Income from SSAT	3.7		0.9		7.7	9.4
Ocean Transportation | Operating segments
Segment results
Total Operating Revenue	410.8		415.4		811.7	813.3
Total Operating Income	42.3		19.7		50.2	29.1
Ocean Transportation | Intersegment Eliminations
Segment results
Total Operating Revenue	11.9		13.8		21.3	25.2
Logistics
Segment results
Total Operating Revenue	113.3		142.5
Logistics | Operating segments
Segment results
Total Operating Revenue	113.3		142.5		226.3	277.0
Total Operating Income	8.9		11.3		14.0	19.4
Logistics | Intersegment Eliminations
Segment results
Total Operating Revenue	$ 12.3		$ 11.5		$ 22.3	$ 21.9