REPORTABLE SEGMENTS (Details) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($) segment
Segment results
Number of reportable segments | segment							2	2
Total Operating Revenue	$ 645.2			$ 572.1			$ 1,683.2	$ 1,662.4
Total Operating Income	98.4			55.2			162.6	103.7
Interest expense, net	(5.7)			(6.2)			(22.5)	(16.9)
Other income (expense), net	2.4			(0.5)			4.5	0.9
Income before Income Taxes	95.1			48.5			144.6	87.7
Income tax expense	(24.2)			(12.3)			(37.1)	(20.6)
Net Income	70.9	$ 32.8	$ 3.8	36.2	$ 18.4	$ 12.5	107.5	67.1
Income from SSAT	7.7			8.4			15.4	17.8
Operating segments
Segment results
Total Operating Revenue	645.2			572.1			1,683.2	1,662.4
Total Operating Income	98.4			55.2			162.6	103.7
Intersegment Eliminations
Segment results
Total Operating Revenue	40.2			28.2			83.8	75.3
Ocean Transportation
Segment results
Income from SSAT	7.7			8.4			15.4	17.8
Ocean Transportation | Operating segments
Segment results
Total Operating Revenue	498.3			437.2			1,310.0	1,250.5
Total Operating Income	86.5			43.9			136.7	73.0
Ocean Transportation | Intersegment Eliminations
Segment results
Total Operating Revenue	22.4			14.0			43.7	39.2
Logistics | Operating segments
Segment results
Total Operating Revenue	146.9			134.9			373.2	411.9
Total Operating Income	11.9			11.3			25.9	30.7
Logistics | Intersegment Eliminations
Segment results
Total Operating Revenue	$ 17.8			$ 14.2			$ 40.1	$ 36.1