SIGNIFICANT ACCOUNTING POLICIES - ACCRUED AND OTHER LIABILITIES (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Intangible assets
Impairment charges related to finite-lived intangible assets	$ 0.0	$ 0.0	$ 0.0
Impairment charges related to indefinite-lived intangible assets and goodwill for impairment	0.0	0.0	$ 0.0
Estimated fair value of inactive vessels		0.0
Accrued and other liabilities
Payroll and vacation liabilities	24.7	23.3
Uninsured claims and related liabilities - short term	15.4	18.4
Employee incentives and other benefits	17.4	8.7
Interest on debt	5.4	5.8
Multi-employer withdrawal liability - short-term (see Note 12)	4.1	4.1
Deferred revenues	5.0	2.9
Pension and post-retirement liabilities - short term (see Note 11)	3.0	2.1
Other liabilities	5.4	10.0
Total	80.4	75.3
Other long-term liabilities
Pension and post-retirement liabilities (see Note 11)	75.1	75.0
Multi-employer withdrawal liabilitiy (see Note 12)	58.4	60.1
Uninsured claims and related liabilities	29.5	27.4
Other long-term liabilities	8.5	9.6
Total	$ 171.5	$ 172.1
Customer Relationships. | Minimum
Intangible assets
Expected useful lives	3 years
Customer Relationships. | Maximum
Intangible assets
Expected useful lives	21 years