REPORTABLE SEGMENTS (Details) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2019 USD ($) segment	Sep. 30, 2018 USD ($)
Segment results
Number of reportable segments | segment							2
Total Operating Revenue	$ 572.1			$ 589.4			$ 1,662.4	$ 1,657.9
Total Operating Income	55.2			58.6			103.7	133.3
Interest expense, net	(6.2)			(4.4)			(16.9)	(14.4)
Other income (expense), net	(0.5)			0.7			0.9	1.9
Income before Income Taxes	48.5			54.9			87.7	120.8
Income tax expense	(12.3)			(13.3)			(20.6)	(32.4)
Net Income	36.2	$ 18.4	$ 12.5	41.6	$ 32.6	$ 14.2	67.1	88.4
Equity in income of Terminal Joint Venture	8.4			9.2			17.8	28.8
Operating segments
Segment results
Total Operating Revenue	572.1			589.4			1,662.4	1,657.9
Total Operating Income	55.2			58.6			103.7	133.3
Intersegment Eliminations
Segment results
Total Operating Revenue	28.2			27.6			75.3	72.7
Ocean Transportation
Segment results
Total Operating Revenue	437.2			437.3			1,250.5	1,223.2
Equity in income of Terminal Joint Venture	8.4			9.2			17.8	28.8
Ocean Transportation | Operating segments
Segment results
Total Operating Revenue	437.2			437.3			1,250.5	1,223.2
Total Operating Income	43.9			48.7			73.0	109.7
Ocean Transportation | Intersegment Eliminations
Segment results
Total Operating Revenue	14.0			15.2			39.2	39.9
Logistics
Segment results
Total Operating Revenue	134.9			152.1			411.9	434.7
Logistics | Operating segments
Segment results
Total Operating Revenue	134.9			152.1			411.9	434.7
Total Operating Income	11.3			9.9			30.7	23.6
Logistics | Intersegment Eliminations
Segment results
Total Operating Revenue	$ 14.2			$ 12.4			$ 36.1	$ 32.8