SIGNIFICANT ACCOUNTING POLICIES - CASH AND ACCOUNTS RECEIVABLE (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents
Outstanding checks in excess of funds on deposits	$ 16.4	$ 18.7
Restricted cash	4.9	0.0
Accounts Receivable
Eligible accounts receivable assigned to CCF	1.0	134.8
Allowance for doubtful accounts
Balance at Beginning of year	4.6	4.2	$ 6.6
Expense (Recovery)	0.8	1.0	(0.3)
Write-offs and Other	(0.6)	(0.6)	(2.1)
Balance at End of Year	4.8	4.6	$ 4.2
Prepaid and Other Assets:
Income tax receivables	26.8	2.6
Prepaid fuel	16.3	14.4
Prepaid insurance and insurance related receivables	12.6	19.3
Prepaid operating expenses	6.8	6.3
Restricted cash - vessel construction obligations	4.9	0.0
Other	7.7	9.0
Total	75.1	51.6
Other Long-term Assets
Alternative minimum tax (AMT) receivable	21.5	50.2
Vessel and equipment spare parts	13.1	12.7
Insurance related receivables	11.2	12.9
Deferred Charges and other	3.7	7.8
Capital construction fund - cash on deposit		0.9
Other long-term assets	$ 49.5	$ 84.5