LEASES - Components of lease expenses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Lease cost
Total operating lease cost	$ 10.3	$ 12.6	$ 20.5	$ 22.6
Interest on lease liabilities			0.6	0.4
Total finance lease cost	1.7	1.4	3.5	2.9
Total lease cost	12.0	14.0	24.0	25.5
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	6.6	7.8	13.0	13.5
Administrative expense | Real estate
Lease cost
Total operating lease cost	3.7	4.8	7.5	9.1
Depreciation and amortization
Lease cost
Amortization right-of-use assets	1.5	1.1	3.0	2.4
Interest Expense
Lease cost
Interest on lease liabilities	$ 0.2	$ 0.3	$ 0.5	$ 0.5