QUARTERLY INFORMATION (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly information by segments
Operating Revenue	$ 494.8	$ 544.3	$ 447.6	$ 398.2	$ 443.5	$ 441.8	$ 436.4	$ 392.5	$ 1,884.9	$ 1,714.2	$ 1,637.2
Operating Income	45.9	71.8	33.7	44.9	49.4	45.0	35.7	9.9	196.3	140.0	100.3
Interest expense	(4.9)	(4.7)	(4.6)	(4.3)	(4.3)	(4.4)	(4.5)	(4.1)	(18.5)	(17.3)	(14.4)
Income before Income Taxes	41.0	67.1	29.1	40.6	45.1	40.6	31.2	5.8	177.8	122.7	85.9
Income tax expense	(14.4)	(25.6)	(19.2)	(15.6)	(17.3)	(19.1)	(13.1)	(2.4)	(74.8)	(51.9)	(32.2)
Net Income	$ 26.6	$ 41.5	$ 9.9	$ 25.0	$ 27.8	$ 21.5	$ 18.1	$ 3.4	$ 103.0	$ 70.8	$ 53.7
Net Income, Earnings Per Share:
Basic Earnings Per Share (in dollars per share)	$ 0.61	$ 0.95	$ 0.23	$ 0.58	$ 0.65	$ 0.50	$ 0.42	$ 0.08	$ 2.37	$ 1.65	$ 1.26
Diluted Earnings Per Share (in dollars per share)	$ 0.60	$ 0.94	$ 0.23	$ 0.57	$ 0.63	$ 0.50	$ 0.42	$ 0.08	$ 2.34	$ 1.63	$ 1.25
Horizon \| Selling, general and administrative costs
Quarterly information by segments
Acquisition Related Selling, General and Administrative Costs									$ (19.0)
Ocean Transportation
Quarterly information by segments
Operating Revenue	$ 401.0	$ 444.8	$ 346.7	$ 305.5	$ 333.2	$ 329.5	$ 321.1	$ 294.6	1,498.0	$ 1,278.4	$ 1,229.4
Operating Income	43.6	68.9	31.4	43.9	46.3	42.6	32.8	9.4	187.8	131.1	94.3
Ocean Transportation \| Selling, general and administrative costs
Quarterly information by segments
Molasses Settlement	(1.9)		(11.4)		(1.0)
Ocean Transportation \| Horizon \| Selling, general and administrative costs
Quarterly information by segments
Acquisition Related Selling, General and Administrative Costs	(1.5)	(5.1)	(12.4)
Logistics
Quarterly information by segments
Operating Revenue	93.8	99.5	100.9	92.7	110.3	112.3	115.3	97.9	386.9	435.8	407.8
Operating Income	$ 2.3	$ 2.9	$ 2.3	$ 1.0	$ 3.1	$ 2.4	$ 2.9	$ 0.5	$ 8.5	$ 8.9	$ 6.0

This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities.

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