LEASES - Components of lease expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Lease cost
Total operating lease cost	$ 33.0	$ 36.0
Interest on lease liabilities	1.1	0.9
Total finance lease cost	6.3	6.3
Total lease cost	39.3	42.3
Impairment charge to right-of-use assets relating to operating leases	3.2	10.0
Impairment charge to right-of-use assets relating to finance leases		0.8
Operations and maintenance | Revenue equipment
Lease cost
Total operating lease cost	24.3	22.2
Administrative expense | Real estate
Lease cost
Total operating lease cost	8.7	13.8
Depreciation and amortization
Lease cost
Amortization right-of-use assets	5.1	5.4
Interest Expense
Lease cost
Interest on lease liabilities	$ 1.2	$ 0.9