REPORTABLE SEGMENTS (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) segment	Mar. 31, 2019 USD ($) segment
Segment results
Number of reportable segments | segment	2	2
Total Operating Revenue	$ 513.9	$ 532.4
Total Operating Income	13.0	17.5
Interest expense, net	(8.6)	(4.6)
Other income (expense), net	0.6	0.6
Income before Income Taxes	5.0	13.5
Income tax expense	(1.2)	(1.0)
Net Income	3.8	12.5
Income from SSAT	4.0	8.5
Operating segments
Segment results
Total Operating Revenue	513.9	532.4
Total Operating Income	13.0	17.5
Intersegment Eliminations
Segment results
Total Operating Revenue	19.4	21.8
Ocean Transportation
Segment results
Total Operating Revenue	400.9	397.9
Income from SSAT	4.0	8.5
Ocean Transportation | Operating segments
Segment results
Total Operating Revenue	400.9	397.9
Total Operating Income	7.9	9.4
Ocean Transportation | Intersegment Eliminations
Segment results
Total Operating Revenue	9.4	11.4
Logistics
Segment results
Total Operating Revenue	113.0	134.5
Logistics | Operating segments
Segment results
Total Operating Revenue	113.0	134.5
Total Operating Income	5.1	8.1
Logistics | Intersegment Eliminations
Segment results
Total Operating Revenue	$ 10.0	$ 10.4