SIGNIFICANT ACCOUNTING POLICIES - IMMATERIAL CORRECTION (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Decrease in employee benefit plan liabilities	$ (75.1)				$ (75.0)				$ (75.1)	$ (75.0)
Decrease in accruals and other liabilities	(80.4)				(75.3)				(80.4)	(75.3)
Net gain (loss) in prior service cost									0.8	24.1	$ 5.1
Increase in retained earnings	381.5				196.6				381.5	196.6
Operating Income (Loss)	25.3	$ 61.8	$ 45.9	$ 16.4	37.2	$ 46.7	$ 36.1	$ 34.6	149.4	154.6	196.3
Income tax expense	(147.0)	21.5	15.6	3.1	10.5	15.4	11.6	11.6	(106.8)	49.1	74.8
Ocean Transportation
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating Income (Loss)	$ 20.7	$ 54.6	$ 39.0	$ 14.5	32.6	$ 43.2	$ 33.9	$ 33.0	$ 128.8	142.7	$ 187.8
Revision of Defined Benefit Plan Liabilities
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Decrease in employee benefit plan liabilities					36.8					36.8
Decrease in accruals and other liabilities					1.6					1.6
Net gain (loss) in prior service cost										22.5
Deferred income taxes					15.0					15.0
Increase in retained earnings					$ 0.9					$ 0.9
Amortization period of unrecognized gains										10 years
Revision of Defined Benefit Plan Liabilities | Ocean Transportation
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating Income (Loss)										$ 1.4
Income tax expense										$ 0.5