REPORTABLE SEGMENTS (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) segment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Segment results
Fiscal Period Duration									371 days	364 days	364 days
Number of reportable segments | segment									2
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, net	(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
Assets	2,015.5				1,669.8				2,015.5	1,669.8	1,401.8
Capital Expenditures									179.4	67.8	27.9
Depreciation and Amortization									97.1	83.4	69.7
Deferred dry-docking amortization									(38.9)	(23.1)	(21.1)
Depreciations and amortizations including deferred dry-docking amortizations									$ 136.0	$ 106.5	$ 90.8
Ocean Transportation
Segment results
Fiscal Period Duration									371 days	364 days	364 days
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
Assets	1,722.2				1,601.0				1,722.2	1,601.0	1,313.9
Capital Expenditures									179.1	67.5	27.8
Depreciation and Amortization									92.6	81.4	66.6
Deferred dry-docking amortization									38.9	23.1	21.1
Ocean Transportation | SSAT
Segment results
Equity in earnings from affiliates	15.8				16.5				15.8	16.5	6.6
Equity method investments	82.4				66.4				82.4	66.4	64.4
Logistics
Segment results
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
Assets	$ 293.3				$ 68.8				293.3	68.8	87.9
Capital Expenditures									0.3	0.3	0.1
Depreciation and Amortization									$ 4.5	$ 2.0	$ 3.1