REPORTABLE SEGMENTS (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) segment	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Segment results
Number of reportable segments | segment									2
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, net	(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
Assets	1,669.8				1,401.8				1,669.8	1,401.8	1,248.3
Capital Expenditures									67.8	27.9	35.2
Depreciation and Amortization									83.4	69.7	69.7
Ocean Transportation
Segment results
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
Assets	1,601.0				1,313.9				1,601.0	1,313.9	1,168.6
Capital Expenditures									67.5	27.8	33.8
Depreciation and Amortization									81.4	66.6	66.4
Ocean Transportation | SSAT
Segment results
Equity in earnings from affiliates	16.5				6.6				16.5	6.6	(2.0)
Equity method investments	66.4				64.4				66.4	64.4	57.6
Logistics
Segment results
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
Assets	$ 68.8				$ 87.9				68.8	87.9	79.7
Capital Expenditures									0.3	0.1	1.4
Depreciation and Amortization									$ 2.0	$ 3.1	$ 3.3