INCOME TAXES - SUMMARY (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
INCOME TAXES
Tax Act, income tax expense (benefit)									$ (2.9)	$ 2.9
Sequestration rate (as a percent)									6.20%
Current:
Federal									$ 0.2	1.5	$ 21.1
State									3.2	2.1	2.2
Foreign									1.3	0.9	0.5
Discrete adjustments related to the Tax Act									(2.9)	2.9
Total									1.8	7.4	23.8
Deferred:
Deferred tax expense									23.3	31.3	24.4
Remeasurement and discrete adjustments related to the Tax Act				$ 2.9	$ 0.2		$ 0.2	$ (3.3)			(154.0)
Total									23.3	31.3	(129.6)
Total income taxes	$ 4.5	$ 12.3	$ 7.3	$ 1.0	6.3	$ 13.3	$ 8.8	$ 10.3	$ 25.1	$ 38.7	$ (105.8)
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%	35.00%
State income tax (as a percent)									4.10%	3.40%	2.60%
Valuation allowance (as a percent)									(0.30%)	(0.70%)	1.40%
Foreign taxes (as a percent)									1.20%	0.60%	0.10%
Remeasurement and discrete adjustments related to Tax Act (as a percent)									(2.70%)	2.00%	(123.00%)
Share-based payments (as a percent)									(0.10%)	0.10%	(1.40%)
Other-net (as a percent)									0.10%	(0.20%)	0.80%
Effective income tax rate , Percent, Total									23.30%	26.20%	(84.50%)
Valuation allowance related to foreign subsidiary losses									$ 0.9	$ (1.1)	$ 1.7
Valuation allowances	10.6				11.5				10.6	11.5
Deferred tax assets:
Operating lease liabilities	63.1								63.1
Pension and post-retirement plans	19.2				20.2				19.2	20.2
Multi-employer withdrawal liabilities	16.1				16.6				16.1	16.6
Federal net operating losses	14.3				15.2				14.3	15.2
State net operating losses	7.3				7.4				7.3	7.4
U.S. State alternative minimum tax credits	6.7				5.9				6.7	5.9
Insurance reserves	5.9				5.6				5.9	5.6
Deferred compensation	5.8				7.0				5.8	7.0
Foreign losses	3.9				5.1				3.9	5.1
Other	2.0				4.7				2.0	4.7
Total deferred tax assets	144.3				87.7				144.3	87.7
Valuation allowance	(10.6)				(11.5)				(10.6)	(11.5)
Total Deferred tax assets, net of valuation allowance	133.7				76.2				133.7	76.2
Deferred tax liabilities:
Basis differences for property and equipment	319.2				302.1				319.2	302.1
Operating lease right of use assets	61.1								61.1
Intangibles	39.7				38.4				39.7	38.4
Lease financing	23.7				26.0				23.7	26.0
Capital Construction Fund	12.5				7.0				12.5	7.0
Investment in SSAT	7.4				11.4				7.4	11.4
Other	7.7				4.0				7.7	4.0
Total deferred tax liabilities	471.3				388.9				471.3	388.9
Deferred tax liability, net , Total	$ 337.6				$ 312.7				$ 337.6	$ 312.7