INCOME TAXES - SUMMARY (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal									$ 21.6	$ 10.5	$ 22.6
State									2.2	(1.3)	2.9
Total									23.8	9.2	25.5
Deferred:
Deferred tax expense									24.4	39.9	49.3
Remeasurement and discrete adjustments related to the Tax Act									(155.0)
Total									(130.6)	39.9	49.3
Total income taxes	$ (147.0)	$ 21.5	$ 15.6	$ 3.1	$ 10.5	$ 15.4	$ 11.6	$ 11.6	$ (106.8)	$ 49.1	$ 74.8
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)									35.00%	35.00%	35.00%
State income tax (as a percent)									2.60%	1.80%	2.50%
Valuation allowance (as a percent)									1.40%	0.30%	1.10%
Foreign taxes (as a percent)									0.10%	0.40%	0.60%
Remeasurement and discrete adjustments related to Tax Act (as a percent)									(123.80%)
Share-based payments (as a percent)									(1.40%)
Other-net (as a percent)									0.80%	0.10%	2.90%
Effective income tax rate , Percent, Total									(85.30%)	37.60%	42.10%
Valuation allowance related to foreign subsidiary losses									$ 1.7	$ 0.4	$ 1.8
Deferred tax assets:
Benefit plans	42.6				63.1				42.6	63.1
Federal net operating losses	21.6				52.1				21.6	52.1
Insurance reserves	6.8				9.9				6.8	9.9
State net operating losses	7.4				7.9				7.4	7.9
Foreign losses	6.6				4.9				6.6	4.9
Alternative minimum tax credits	4.2				31.0				4.2	31.0
Allowance for doubtful accounts	0.9				1.3				0.9	1.3
Reserves					2.4					2.4
Other	1.9				3.5				1.9	3.5
Total deferred tax assets	92.0				176.1				92.0	176.1
Valuation allowance	(13.0)				(11.9)				(13.0)	(11.9)
Total Deferred tax assets, net of valuation allowance	79.0				164.2				79.0	164.2
Deferred tax liabilities:
Basis differences for property and equipment	254.4				339.0				254.4	339.0
Capital Construction Fund	54.2				115.8				54.2	115.8
Intangibles	36.4				53.8				36.4	53.8
Deferred revenue	6.9				9.5				6.9	9.5
Joint ventures and other investments	9.6				9.9				9.6	9.9
Reserves - Liabilities	2.7								2.7
Total deferred tax liabilities	364.2				528.0				364.2	528.0
Deferred tax liability, net , Total	$ 285.2				$ 363.8				$ 285.2	$ 363.8