QUARTERLY INFORMATION (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly information by segments
Operating Revenue	$ 516.1	$ 543.9	$ 512.5	$ 474.4	$ 519.3	$ 500.4	$ 467.7	$ 454.2	$ 2,046.9	$ 1,941.6	$ 1,884.9
Operating Income	25.3	61.8	45.9	16.4	37.2	46.7	36.1	34.6	149.4	154.6	196.3
Interest expense	(5.4)	(6.2)	(6.3)	(6.3)	(6.7)	(6.0)	(6.5)	(4.9)	(24.2)	(24.1)	(18.5)
Income before Income Taxes	19.9	55.6	39.6	10.1	30.5	40.7	29.6	29.7	125.2	130.5	177.8
Income tax expense	147.0	(21.5)	(15.6)	(3.1)	(10.5)	(15.4)	(11.6)	(11.6)	106.8	(49.1)	(74.8)
Net Income	$ 166.9	$ 34.1	$ 24.0	$ 7.0	20.0	$ 25.3	$ 18.0	$ 18.1	$ 232.0	$ 81.4	$ 103.0
Molasses Settlement					$ (0.7)
Net Income, Earnings Per Share:
Basic Earnings Per-Share: (in dollars per share)	$ 3.93	$ 0.79	$ 0.56	$ 0.16	$ 0.47	$ 0.59	$ 0.42	$ 0.42	$ 5.41	$ 1.89	$ 2.37
Diluted Earnings Per-Share: (in dollars per share)	$ 3.90	$ 0.79	$ 0.55	$ 0.16	$ 0.46	$ 0.59	$ 0.42	$ 0.41	$ 5.37	$ 1.87	$ 2.34
Span Alaska
Quarterly information by segments
Acquisition Related Selling, General and Administrative Costs										$ (3.0)
Horizon
Quarterly information by segments
Acquisition Related Selling, General and Administrative Costs					$ (0.2)	$ (2.6)	$ (0.1)	$ (0.1)			$ (19.0)
Ocean Transportation
Quarterly information by segments
Operating Revenue	$ 389.9	$ 419.2	$ 392.7	$ 370.0	406.1	398.0	370.9	366.1	$ 1,571.8	1,541.1	1,498.0
Operating Income	20.7	54.6	39.0	14.5	32.6	43.2	33.9	33.0	128.8	142.7	187.8
Logistics
Quarterly information by segments
Operating Revenue	126.2	124.7	119.8	104.4	113.2	102.4	96.8	88.1	475.1	400.5	386.9
Operating Income	$ 4.6	$ 7.2	$ 6.9	$ 1.9	$ 4.6	$ 3.5	$ 2.2	$ 1.6	$ 20.6	$ 11.9	$ 8.5