QUARTERLY INFORMATION (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly information by segments
Total Operating Revenue	$ 540.7	$ 572.1	$ 557.9	$ 532.4	$ 564.9	$ 589.4	$ 557.1	$ 511.4	$ 2,203.1	$ 2,222.8	$ 2,046.9
Operating Income	25.4	55.2	31.0	17.5	30.5	58.6	46.0	28.7	129.1	163.8	147.3
Interest expense, net	(5.6)	(6.2)	(6.1)	(4.6)	(4.3)	(4.4)	(5.0)	(5.0)	(22.5)	(18.7)	(24.2)
Other income (expense), net	0.3	(0.5)	0.8	0.6	0.7	0.7	0.4	0.8	1.2	2.6	2.1
Income before Income Taxes	20.1	48.5	25.7	13.5	26.9	54.9	41.4	24.5	107.8	147.7	125.2
Income tax expense	(4.5)	(12.3)	(7.3)	(1.0)	(6.3)	(13.3)	(8.8)	(10.3)	(25.1)	(38.7)	105.8
Net Income	$ 15.6	$ 36.2	$ 18.4	12.5	20.6	$ 41.6	32.6	14.2	$ 82.7	$ 109.0	231.0
Income taxes - Discrete adjustments related to the Tax Act				$ 2.9	$ 0.2		$ 0.2	$ (3.3)			$ (154.0)
Net Income, Earnings Per Share:
Basic Earnings Per-Share: (in dollars per share)	$ 0.36	$ 0.84	$ 0.43	$ 0.29	$ 0.48	$ 0.97	$ 0.76	$ 0.33	$ 1.93	$ 2.55	$ 5.38
Diluted Earnings Per-Share: (in dollars per share)	$ 0.36	$ 0.84	$ 0.43	$ 0.29	$ 0.48	$ 0.97	$ 0.76	$ 0.33	$ 1.91	$ 2.53	$ 5.35
Ocean Transportation
Quarterly information by segments
Total Operating Revenue	$ 416.1	$ 437.2	$ 415.4	$ 397.9	$ 418.1	$ 437.3	$ 406.6	$ 379.3
Operating Income	17.8	43.9	19.7	9.4	21.4	48.7	36.5	24.5
Logistics
Quarterly information by segments
Total Operating Revenue	124.6	134.9	142.5	134.5	146.8	152.1	150.5	132.1
Operating Income	$ 7.6	$ 11.3	$ 11.3	$ 8.1	$ 9.1	$ 9.9	$ 9.5	$ 4.2