INCOME TAXES - SUMMARY (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
INCOME TAXES
Income taxes - Discrete adjustments related to the Tax Act (1)			$ (2.9)
Current:
Federal	$ 181.2		0.2
State	35.6	$ 8.7	3.2
Foreign	2.5	1.4	1.3
Discrete adjustments related to the Tax Act			(2.9)
Total	219.3	10.1	1.8
Deferred:
Deferred tax expense	24.6	55.8	23.3
Total income taxes	$ 243.9	$ 65.9	$ 25.1
Difference of Income tax expense from amounts computed by applying the statutory federal rate to income from continuing operations before income taxes
Computed federal income tax expense (as a percent)	21.00%	21.00%	21.00%
State income tax (as a percent)	3.10%	3.50%	4.10%
Foreign-derived intangible income (FDII) (as a percent)	(2.50%)
Valuation allowance (as a percent)	(0.30%)	(0.20%)	(0.30%)
Foreign taxes (as a percent)	0.20%	0.60%	1.20%
Remeasurement and discrete adjustments related to Tax Act (as a percent)			(2.70%)
Share-based payments (as a percent)	(0.20%)	(0.50%)	(0.10%)
Other-net (as a percent)	(0.50%)	1.00%	0.10%
Effective income tax rate , Percent, Total	20.80%	25.40%	23.30%
Valuation allowance related to foreign subsidiary losses	$ 4.7	$ 0.6	$ 0.9
Valuation allowances	5.3	10.0
Deferred tax assets:
Operating lease liabilities	111.7	63.5
Multi-employer withdrawal liabilities	13.8	15.6
Pension and post-retirement plans	11.1	21.7
Deferred compensation	10.4	8.2
Insurance reserves	6.0	5.3
State net operating losses	5.4	7.5
Other	17.8	15.7
Total deferred tax assets	176.2	137.5
Valuation allowance	(5.3)	(10.0)
Total Deferred tax assets, net of valuation allowance	170.9	127.5
Deferred tax liabilities:
Basis differences for property and equipment	423.5	372.4
Operating lease right of use assets	109.3	61.5
Intangibles	41.3	40.5
Investment in SSAT	12.4	7.4
Capital Construction Fund	1.3	6.7
Lease financing		22.0
Other	6.0	6.6
Total deferred tax liabilities	593.8	517.1
Deferred tax liability, net , Total	$ 422.9	$ 389.6