REPORTABLE SEGMENTS (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment results
Number of reportable segments | segment									2
Fiscal Period Duration									364 days	364 days	371 days
Total Operating Revenue	$ 564.9	$ 589.4	$ 557.1	$ 511.4	$ 516.1	$ 543.9	$ 512.5	$ 474.4	$ 2,222.8	$ 2,046.9	$ 1,941.6
Operating Income	30.5	58.6	46.0	28.7	24.8	58.3	47.0	17.2	163.8	147.3	156.7
Interest expense, net	(4.3)	(4.4)	(5.0)	(5.0)	(5.4)	(6.2)	(6.3)	(6.3)	(18.7)	(24.2)	(24.1)
Other income (expense), net	0.7	0.7	0.4	0.8	0.5	3.5	(1.1)	(0.8)	2.6	2.1	(2.1)
Income before Income Taxes	26.9	54.9	41.4	24.5	19.9	55.6	39.6	10.1	147.7	125.2	130.5
Income tax expense	(6.3)	(13.3)	(8.8)	(10.3)	146.0	(21.5)	(15.6)	(3.1)	(38.7)	105.8	(49.1)
Net Income	20.6	41.6	32.6	14.2	165.9	34.1	24.0	7.0	109.0	231.0	81.4
Depreciation and Amortization									94.4	101.2	97.1
Deferred dry-docking amortization									(37.4)	(46.2)	(38.9)
Total Depreciation and Amortization									131.8	147.4	136.0
Assets	2,430.4				2,251.6				2,430.4	2,251.6
Equity in income of Terminal Joint Venture									36.8	28.2	15.8
Operating segments
Segment results
Total Operating Revenue									2,222.8	2,046.9	1,941.6
Operating Income									163.8	147.3	156.7
Capital Expenditures									401.2	307.0	179.4
Depreciation and Amortization									94.4	101.2	97.1
Assets	2,430.4				2,251.6				2,430.4	2,251.6
Intersegment Eliminations
Segment results
Total Operating Revenue									$ 95.4	$ 81.3	$ 62.0
Ocean Transportation
Segment results
Fiscal Period Duration									364 days	364 days	371 days
Total Operating Revenue	418.1	437.3	406.6	379.3	389.9	419.2	392.7	370.0
Operating Income	21.4	48.7	36.5	24.5	20.1	51.0	40.0	15.3
Ocean Transportation | SSAT
Segment results
Equity in income of Terminal Joint Venture									$ 36.8	$ 28.2	$ 15.8
Equity method investments	87.0				93.2				87.0	93.2
Ocean Transportation | Operating segments
Segment results
Total Operating Revenue									1,641.3	1,571.8	1,541.1
Operating Income									131.1	126.4	144.5
Capital Expenditures									385.4	305.3	179.1
Depreciation and Amortization									87.0	93.3	92.6
Deferred dry-docking amortization									37.4	46.2	38.9
Assets	2,071.6				1,941.5				2,071.6	1,941.5
Logistics
Segment results
Total Operating Revenue	146.8	152.1	150.5	132.1	126.2	124.7	119.8	104.4
Operating Income	9.1	$ 9.9	$ 9.5	$ 4.2	4.7	$ 7.3	$ 7.0	$ 1.9
Logistics | Operating segments
Segment results
Total Operating Revenue									581.5	475.1	400.5
Operating Income									32.7	20.9	12.2
Capital Expenditures									15.8	1.7	0.3
Depreciation and Amortization									7.4	7.9	$ 4.5
Assets	$ 358.8				$ 310.1				$ 358.8	$ 310.1