QUARTERLY INFORMATION (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly information by segments
Total Operating Revenue	$ 564.9	$ 589.4	$ 557.1	$ 511.4	$ 516.1	$ 543.9	$ 512.5	$ 474.4	$ 2,222.8	$ 2,046.9	$ 1,941.6
Operating Income	30.5	58.6	46.0	28.7	24.8	58.3	47.0	17.2	163.8	147.3	156.7
Interest expense, net	(4.3)	(4.4)	(5.0)	(5.0)	(5.4)	(6.2)	(6.3)	(6.3)	(18.7)	(24.2)	(24.1)
Other income (expense), net	0.7	0.7	0.4	0.8	0.5	3.5	(1.1)	(0.8)	2.6	2.1	(2.1)
Income before Income Taxes	26.9	54.9	41.4	24.5	19.9	55.6	39.6	10.1	147.7	125.2	130.5
Income tax expense	(6.3)	(13.3)	(8.8)	(10.3)	146.0	(21.5)	(15.6)	(3.1)	(38.7)	105.8	(49.1)
Net Income	20.6	$ 41.6	32.6	14.2	$ 165.9	34.1	$ 24.0	$ 7.0	$ 109.0	231.0	$ 81.4
Remeasurement and discrete adjustments related to the Tax Act	$ (0.2)		$ (0.2)	$ 3.3		$ (154.0)				$ (154.0)
Net Income, Earnings Per Share:
Basic Earnings Per-Share: (in dollars per share)	$ 0.48	$ 0.97	$ 0.76	$ 0.33	$ 3.90	$ 0.79	$ 0.56	$ 0.16	$ 2.55	$ 5.38	$ 1.89
Diluted Earnings Per-Share: (in dollars per share)	$ 0.48	$ 0.97	$ 0.76	$ 0.33	$ 3.88	$ 0.79	$ 0.55	$ 0.16	$ 2.53	$ 5.35	$ 1.87
Ocean Transportation
Quarterly information by segments
Total Operating Revenue	$ 418.1	$ 437.3	$ 406.6	$ 379.3	$ 389.9	$ 419.2	$ 392.7	$ 370.0
Operating Income	21.4	48.7	36.5	24.5	20.1	51.0	40.0	15.3
Logistics
Quarterly information by segments
Total Operating Revenue	146.8	152.1	150.5	132.1	126.2	124.7	119.8	104.4
Operating Income	$ 9.1	$ 9.9	$ 9.5	$ 4.2	$ 4.7	$ 7.3	$ 7.0	$ 1.9