LEASES - Components of lease expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Lease cost
Total operating lease cost	$ 47.3	$ 43.8	$ 42.2
Interest on lease liabilities	1.1	1.1	1.2
Total finance lease cost	7.6	7.5	7.9
Total lease cost	54.9	51.3	50.1
Operations and maintenance | Revenue equipment
Lease cost
Total operating lease cost	27.4	27.7	25.5
Administrative expense | Real estate
Lease cost
Total operating lease cost	16.3	13.6	14.9
Operations and maintenance, and Administrative
Lease cost
Variable Lease, Cost	2.0	1.3	0.9
Short-term Lease, Cost	1.6	1.2	0.9
Depreciation and amortization
Lease cost
Amortization right-of-use assets	6.5	6.4	6.7
Interest Expense
Lease cost
Interest on lease liabilities	$ 1.1	$ 1.1	$ 1.2