REPORTABLE SEGMENTS (Details) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Jun. 30, 2019 USD ($) segment	Jun. 30, 2018 USD ($)
Segment results
Number of reportable segments | segment					2
Total Operating Revenue	$ 557.9		$ 557.1		$ 1,090.3	$ 1,068.5
Total Operating Income	31.0		46.0		48.5	74.7
Interest expense, net	(6.1)		(5.0)		(10.7)	(10.0)
Other income (expense), net	0.8		0.4		1.4	1.2
Income before Income Taxes	25.7		41.4		39.2	65.9
Income tax expense	(7.3)		(8.8)		(8.3)	(19.1)
Net Income	18.4	$ 12.5	32.6	$ 14.2	30.9	46.8
Equity in income of Terminal Joint Venture	0.9		9.1		9.4	19.6
Operating segments
Segment results
Total Operating Revenue	557.9		557.1		1,090.3	1,068.5
Total Operating Income	31.0		46.0		48.5	74.7
Intersegment Eliminations
Segment results
Total Operating Revenue	25.3		26.8		47.1	45.1
Ocean Transportation
Segment results
Total Operating Revenue	415.4		406.6		813.3	785.9
Ocean Transportation | Operating segments
Segment results
Total Operating Revenue	415.4		406.6		813.3	785.9
Total Operating Income	19.7		36.5		29.1	61.0
Logistics
Segment results
Total Operating Revenue	142.5		150.5		277.0	282.6
Logistics | Operating segments
Segment results
Total Operating Revenue	142.5		150.5		277.0	282.6
Total Operating Income	$ 11.3		$ 9.5		$ 19.4	$ 13.7