QUARTERLY INFORMATION (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly information by segments
Operating Revenue	$ 519.3	$ 500.4	$ 467.7	$ 454.2	$ 494.8	$ 544.3	$ 447.6	$ 398.2	$ 1,941.6	$ 1,884.9	$ 1,714.2
Operating Income	36.3	46.2	36.1	34.6	45.9	71.8	33.7	44.9	153.2	196.3	140.0
Interest expense	(6.7)	(6.0)	(6.5)	(4.9)	(4.9)	(4.7)	(4.6)	(4.3)	(24.1)	(18.5)	(17.3)
Income before Income Taxes	29.6	40.2	29.6	29.7	41.0	67.1	29.1	40.6	129.1	177.8	122.7
Income tax expense	(10.2)	(15.2)	(11.6)	(11.6)	(14.4)	(25.6)	(19.2)	(15.6)	(48.6)	(74.8)	(51.9)
Net Income	19.4	$ 25.0	$ 18.0	$ 18.1	26.6	$ 41.5	9.9	$ 25.0	$ 80.5	$ 103.0	$ 70.8
Molasses Settlement	$ (0.7)				$ (1.9)		$ (11.4)
Net Income, Earnings Per Share:
Basic Earnings Per-Share: (in dollars per share)	$ 0.45	$ 0.58	$ 0.42	$ 0.42	$ 0.61	$ 0.95	$ 0.23	$ 0.58	$ 1.87	$ 2.37	$ 1.65
Diluted Earnings Per-Share: (in dollars per share)	$ 0.44	$ 0.58	$ 0.42	$ 0.41	$ 0.60	$ 0.94	$ 0.23	$ 0.57	$ 1.85	$ 2.34	$ 1.63
Span Alaska
Quarterly information by segments
Acquisition Related Selling, General and Administrative Costs	$ (0.2)	$ (2.6)	$ (0.1)	$ (0.1)					$ (3.0)
Horizon
Quarterly information by segments
Acquisition Related Selling, General and Administrative Costs					$ (1.5)	$ (5.1)	$ (12.4)			$ (19.0)
Ocean Transportation
Quarterly information by segments
Operating Revenue	406.1	398.0	370.9	366.1	401.0	444.8	346.7	$ 305.5	1,541.1	1,498.0	$ 1,278.4
Operating Income	31.7	42.7	33.9	33.0	43.6	68.9	31.4	43.9	141.3	187.8	131.1
Logistics
Quarterly information by segments
Operating Revenue	113.2	102.4	96.8	88.1	93.8	99.5	100.9	92.7	400.5	386.9	435.8
Operating Income	$ 4.6	$ 3.5	$ 2.2	$ 1.6	$ 2.3	$ 2.9	$ 2.3	$ 1.0	$ 11.9	$ 8.5	$ 8.9