INCOME TAXES - SUMMARY (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
INCOME TAXES
Tax Act, income tax expense (benefit)									$ 2.9
Sequestration rate (as a percent)									6.20%
Non-cash adjustment									$ 2.9
Current:
Federal									2.4	$ 21.6	$ 10.5
State									2.1	2.2	(1.3)
Discrete adjustments related to the Tax Act									2.9
Total									7.4	23.8	9.2
Deferred:
Deferred tax expense									31.3	24.4	39.9
Remeasurement and discrete adjustments related to the Tax Act	$ (0.2)		$ (0.2)	$ 3.3		$ (154.0)				(154.0)
Total									31.3	(129.6)	39.9
Total income taxes	6.3	$ 13.3	$ 8.8	$ 10.3	$ (146.0)	$ 21.5	$ 15.6	$ 3.1	$ 38.7	$ (105.8)	$ 49.1
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%	35.00%	35.00%
State income tax (as a percent)									3.40%	2.60%	1.80%
Valuation allowance (as a percent)									(0.70%)	1.40%	0.30%
Foreign taxes (as a percent)									0.60%	0.10%	0.40%
Remeasurement and discrete adjustments related to Tax Act (as a percent)									2.00%	(123.00%)
Share-based payments (as a percent)									0.10%	(1.40%)
Other-net (as a percent)									(0.20%)	0.80%	0.10%
Effective income tax rate , Percent, Total									26.20%	(84.50%)	37.60%
Valuation allowance related to foreign subsidiary losses									$ (1.1)	$ 1.7	$ 0.4
Valuation allowances	11.5				13.0				11.5	13.0
Deferred tax assets:
Benefit plans	45.6				44.2				45.6	44.2
Federal net operating losses	15.2				21.6				15.2	21.6
Insurance reserves	5.6				6.8				5.6	6.8
State net operating losses	7.4				7.4				7.4	7.4
Foreign losses	5.1				6.6				5.1	6.6
State alternative minimum tax credits	5.9				4.2				5.9	4.2
Allowance for doubtful accounts	1.1				0.9				1.1	0.9
Other	1.8				1.9				1.8	1.9
Total deferred tax assets	87.7				93.6				87.7	93.6
Valuation allowance	(11.5)				(13.0)				(11.5)	(13.0)
Total Deferred tax assets, net of valuation allowance	76.2				80.6				76.2	80.6
Deferred tax liabilities:
Basis differences for property and equipment	302.1				254.4				302.1	254.4
Lease financing	26.0								26.0
Capital Construction Fund	7.0				54.2				7.0	54.2
Intangibles	38.4				36.4				38.4	36.4
Deferred revenue	3.0				6.9				3.0	6.9
Terminal Joint Venture investment	11.4				9.6				11.4	9.6
Reserves - Liabilities	1.0				2.7				1.0	2.7
Total deferred tax liabilities	388.9				364.2				388.9	364.2
Deferred tax liability, net , Total	$ 312.7				$ 283.6				$ 312.7	$ 283.6