REPORTABLE SEGMENTS (Details) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)
Segment results
Number of segments | segment							2
Operating Revenue	$ 589.4			$ 543.9			$ 1,657.9	$ 1,530.8
Operating Income	58.6			58.3			133.3	122.5
Interest expense, net	(4.4)			(6.2)			(14.4)	(18.8)
Other income (expense), net	0.7			3.5			1.9	1.6
Income before Income Taxes	54.9			55.6			120.8	105.3
Income taxes	(13.3)			(21.5)			(32.4)	(40.2)
Net Income	41.6	$ 32.6	$ 14.2	34.1	$ 24.0	$ 7.0	88.4	65.1
Equity in income of Terminal Joint Venture	9.2			7.5			28.8	19.3
Operating segments
Segment results
Operating Income	58.6			58.3			133.3	122.5
Intersegment Eliminations
Segment results
Operating Revenue	27.6			24.1			72.7	62.7
Ocean Transportation | Operating segments
Segment results
Operating Revenue	437.3			419.2			1,223.2	1,181.9
Operating Income	48.7			51.0			109.7	106.3
Logistics | Operating segments
Segment results
Operating Revenue	152.1			124.7			434.7	348.9
Operating Income	$ 9.9			$ 7.3			$ 23.6	$ 16.2