LEASES - Components of lease expenses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Lease cost
Total operating lease cost	$ 10.2	$ 10.0
Interest on lease liabilities	0.3	0.2
Total finance lease cost	1.8	1.5
Total lease cost	12.0	11.5
Impairment charge to right-of-use assets relating to operating leases	3.2	0.0
Operations and maintenance | Revenue equipment
Lease cost
Total operating lease cost	6.4	5.8
Administrative expense | Real estate
Lease cost
Total operating lease cost	3.8	4.2
Depreciation and amortization
Lease cost
Amortization right-of-use assets	1.5	1.3
Interest Expense
Lease cost
Interest on lease liabilities	$ 0.3	$ 0.2