REPORTABLE SEGMENTS (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment results
Number of segments | segment									2
Fiscal Period Duration									364 days	364 days
Total Operating Revenue	$ 540.7	$ 572.1	$ 557.9	$ 532.4	$ 564.9	$ 589.4	$ 557.1	$ 511.4	$ 2,203.1	$ 2,222.8	$ 2,046.9
Operating Income	25.4	55.2	31.0	17.5	30.5	58.6	46.0	28.7	129.1	163.8	147.3
Interest expense, net	(5.6)	(6.2)	(6.1)	(4.6)	(4.3)	(4.4)	(5.0)	(5.0)	(22.5)	(18.7)	(24.2)
Other income (expense), net	0.3	(0.5)	0.8	0.6	0.7	0.7	0.4	0.8	1.2	2.6	2.1
Income before Income Taxes	20.1	48.5	25.7	13.5	26.9	54.9	41.4	24.5	107.8	147.7	125.2
Income taxes	(4.5)	(12.3)	(7.3)	(1.0)	(6.3)	(13.3)	(8.8)	(10.3)	(25.1)	(38.7)	105.8
Net Income	15.6	36.2	18.4	12.5	20.6	41.6	32.6	14.2	82.7	109.0	231.0
Depreciation and Amortization									100.4	94.4	101.2
Deferred dry-docking amortization									34.3	37.4	46.2
Total Depreciation and Amortization									134.7	131.8	147.4
Assets	2,845.4				2,430.4				2,845.4	2,430.4
Income from SSAT									20.8	36.8	28.2
Ocean Transportation
Segment results
Total Operating Revenue	416.1	437.2	415.4	397.9	418.1	437.3	406.6	379.3
Operating Income	17.8	43.9	19.7	9.4	21.4	48.7	36.5	24.5
Ocean Transportation | SSAT
Segment results
Income from SSAT									20.8	36.8	28.2
Equity method investments	76.2				87.0				76.2	87.0
Logistics
Segment results
Total Operating Revenue	124.6	134.9	142.5	134.5	146.8	152.1	150.5	132.1
Operating Income	7.6	$ 11.3	$ 11.3	$ 8.1	9.1	$ 9.9	$ 9.5	$ 4.2
Operating segments
Segment results
Total Operating Revenue									2,203.1	2,222.8	2,046.9
Operating Income									129.1	163.8	147.3
Capital Expenditures									310.3	401.2	307.0
Depreciation and Amortization									100.4	94.4	101.2
Assets	2,845.4				2,430.4				2,845.4	2,430.4
Operating segments | Ocean Transportation
Segment results
Total Operating Revenue									1,666.6	1,641.3	1,571.8
Operating Income									90.8	131.1	126.4
Capital Expenditures									294.5	385.4	305.3
Depreciation and Amortization									93.6	87.0	93.3
Deferred dry-docking amortization									34.3	37.4	46.2
Assets	2,424.5				2,071.6				2,424.5	2,071.6
Operating segments | Logistics
Segment results
Total Operating Revenue									536.5	581.5	475.1
Operating Income									38.3	32.7	20.9
Capital Expenditures									15.8	15.8	1.7
Depreciation and Amortization									6.8	7.4	7.9
Assets	$ 420.9				$ 358.8				420.9	358.8
Intersegment Eliminations
Segment results
Total Operating Revenue									102.3	95.4	81.3
Intersegment Eliminations | Ocean Transportation
Segment results
Total Operating Revenue									52.8	51.7	40.3
Intersegment Eliminations | Logistics
Segment results
Total Operating Revenue									$ 49.5	$ 43.7	$ 41.0