SIGNIFICANT ACCOUNTING POLICIES - CASH AND ACCOUNTS RECEIVABLE (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash and Cash Equivalents
Outstanding checks in excess of funds on deposits	$ 19.9	$ 13.8
Restricted cash	5.3	7.2
Allowance for doubtful accounts
Balance at Beginning of year	4.3	4.8	$ 4.6
Expense	2.9	0.6	0.8
Write-offs and Other	(0.9)	(1.1)	(0.6)
Balance at End of Year	6.3	4.3	$ 4.8
Prepaid and Other Assets:
Prepaid fuel	10.8	13.7
Prepaid insurance and insurance related receivables	7.5	13.4
Prepaid operating expenses	5.4	5.7
Restricted cash - vessel construction obligations	5.3	7.2
Income tax receivables	0.3	12.8
Other	8.8	9.7
Total	38.1	62.5
Other Long-term Assets
Vessel and equipment spare parts	11.5	12.4
Insurance related receivables	10.5	10.6
Deferred loan fees	2.7	2.1
Cloud computing software costs	2.4
Income tax receivables	2.1	11.5
Other	3.8	1.2
Total	$ 33.0	$ 37.8