SIGNIFICANT ACCOUNTING POLICIES - CASH AND ACCOUNTS RECEIVABLE (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents
Outstanding checks in excess of funds on deposits	$ 18.7	$ 21.3
Accounts Receivable
Eligible accounts receivable assigned to CCF	134.8	174.7
Allowance for doubtful accounts
Balance at Beginning of year	4.2	6.6	$ 5.0
Expense (Recovery)	1.0	(0.3)	2.0
Write-offs and Other	(0.6)	(2.1)	(0.4)
Balance at End of Year	4.6	4.2	$ 6.6
Prepaid and Other Assets:
Income tax receivable	2.6	23.4
Insurance	15.2	17.6
Prepaid fuel	14.4	11.5
Other	19.4	18.3
Total	51.6	70.8
Other Long-term Assets
Alternative minimum tax (AMT) receivable	50.2
Deferred charges and other	20.7	17.7
Vessel and equipment spare parts	12.7	11.4
Net Balance of Amounts Accrued for Capital Construction Fund	0.9	31.2
Other long-term assets	$ 84.5	$ 60.3