INCOME TAXES - SUMMARY (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
INCOME TAXES
Income taxes - Discrete adjustments related to the Tax Act (1)								$ 2.9		$ (2.9)	$ 2.9
Current:
Federal										0.2	1.5
State									$ 8.7	3.2	2.1
Foreign									1.4	1.3	0.9
Discrete adjustments related to the Tax Act								2.9		(2.9)	2.9
Total									10.1	1.8	7.4
Deferred:
Deferred tax expense									55.8	23.3	31.3
Total income taxes	$ 28.8	$ 24.2	$ 11.7	$ 1.2	$ 4.5	$ 12.3	$ 7.3	$ 1.0	$ 65.9	$ 25.1	$ 38.7
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%	35.00%
State income tax (as a percent)									3.50%	4.10%	3.40%
Valuation allowance (as a percent)									(0.20%)	(0.30%)	(0.70%)
Foreign taxes (as a percent)									0.60%	1.20%	0.60%
Remeasurement and discrete adjustments related to Tax Act (as a percent)										(2.70%)	2.00%
Share-based payments (as a percent)									(0.50%)	(0.10%)	0.10%
Other-net (as a percent)									1.00%	0.10%	(0.20%)
Effective income tax rate , Percent, Total									25.40%	23.30%	26.20%
Valuation allowance related to foreign subsidiary losses									$ 0.6	$ 0.9	$ (1.1)
Valuation allowances	10.0				10.6				10.0	10.6
Deferred tax assets:
Operating lease liabilities	63.5				63.1				63.5	63.1
Pension and post-retirement plans	21.7				19.2				21.7	19.2
Multi-employer withdrawal liabilities	15.6				16.1				15.6	16.1
Federal net operating losses	0.9				14.3				0.9	14.3
State net operating losses	7.5				7.3				7.5	7.3
U.S. State alternative minimum tax credits	4.4				6.7				4.4	6.7
Insurance reserves	5.3				5.9				5.3	5.9
Deferred compensation	8.2				5.8				8.2	5.8
Other	10.4				5.9				10.4	5.9
Total deferred tax assets	137.5				144.3				137.5	144.3
Valuation allowance	(10.0)				(10.6)				(10.0)	(10.6)
Total Deferred tax assets, net of valuation allowance	127.5				133.7				127.5	133.7
Deferred tax liabilities:
Basis differences for property and equipment	372.4				319.2				372.4	319.2
Operating lease right of use assets	61.5				61.1				61.5	61.1
Intangibles	40.5				39.7				40.5	39.7
Lease financing	22.0				23.7				22.0	23.7
Capital Construction Fund	6.7				12.5				6.7	12.5
Investment in SSAT	7.4				7.4				7.4	7.4
Other	6.6				7.7				6.6	7.7
Total deferred tax liabilities	517.1				471.3				517.1	471.3
Deferred tax liability, net , Total	$ 389.6				$ 337.6				$ 389.6	$ 337.6