FAIR VALUE OF FINANCIAL INSTRUMENTS (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
Carrying value
Cash and Cash Equivalents
Cash and cash equivalents	19.8	13.9
CCF - cash on deposit	0.9	31.2
Variable rate debt	205.0	55.0
Fixed rate debt	652.1	683.9
Fair Value Measurement
Cash and Cash Equivalents
Cash and cash equivalents	19.8	13.9
CCF - cash on deposit	0.9	31.2
Variable rate debt	205.0	55.0
Fixed rate debt	651.4	685.2
Quoted Prices in Active Markets (Level 1)
Cash and Cash Equivalents
Cash and cash equivalents	19.8	13.9
Significant Observable Inputs (Level 2)
Cash and Cash Equivalents
CCF - cash on deposit	0.9	31.2
Variable rate debt	205.0	55.0
Fixed rate debt	$ 651.4	$ 685.2