SIGNIFICANT ACCOUNTING POLICIES - (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Decrease in employee benefit plan liabilities	$ (66.5)		$ (66.5)		$ (71.7)
Decrease in accruals and other liabilities	(82.4)		(82.4)		(75.3)
Net gain (loss) in prior service cost	0.7	$ 23.4	0.8	$ 24.1
Increase in retained earnings	223.4		223.4		196.6
Operating Income (Loss)	61.8	46.7	124.1	117.4
Income tax expense	21.5	15.4	40.2	38.6
Operating segments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating Income (Loss)	61.8	46.7	124.1	117.4
Operating segments | Ocean Transportation
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating Income (Loss)	$ 54.6	$ 43.2	108.1	$ 110.1
Revision of Defined Benefit Plan Liabilities
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Decrease in employee benefit plan liabilities					36.8
Decrease in accruals and other liabilities					1.6
Net gain (loss) in prior service cost					22.5
Deferred income taxes					15.0
Increase in retained earnings					$ 0.9
Amortization period of unrecognized gains					10 years
Revision of Defined Benefit Plan Liabilities | Operating segments | Ocean Transportation
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating Income (Loss)			0.5
Income tax expense			$ 0.2