REPORTABLE SEGMENTS (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) segment	Jun. 30, 2017 USD ($)
Segment results
Number of segments | segment			2
Operating Revenue	$ 557.1	$ 512.5	$ 1,068.5	$ 986.9
Operating Income	46.0	47.0	74.7	64.2
Interest expense, net	(5.0)	(6.3)	(10.0)	(12.6)
Other income (expense), net	0.4	(1.1)	1.2	(1.9)
Income before Income Taxes	41.4	39.6	65.9	49.7
Income taxes	(8.8)	(15.6)	(19.1)	(18.7)
Net Income	32.6	24.0	46.8	31.0
Equity in income of Terminal Joint Venture	9.1	6.9	19.6	11.8
Operating segments
Segment results
Operating Income	46.0	47.0	74.7	64.2
Intersegment Eliminations
Segment results
Operating Revenue	26.8	21.7	45.1	38.6
Ocean Transportation | Operating segments
Segment results
Operating Revenue	406.6	392.7	785.9	762.7
Operating Income	36.5	40.0	61.0	55.3
Logistics | Operating segments
Segment results
Operating Revenue	150.5	119.8	282.6	224.2
Operating Income	$ 9.5	$ 7.0	$ 13.7	$ 8.9