INCOME TAXES - SUMMARY (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current:
Federal									$ 10.0	$ 22.6	$ 45.5
State									(1.3)	2.9	3.7
Total									8.7	25.5	49.2
Deferred:									39.9	49.3	2.7
Total income taxes expense	$ 10.2	$ 15.2	$ 11.6	$ 11.6	$ 14.4	$ 25.6	$ 19.2	$ 15.6	$ 48.6	$ 74.8	$ 51.9
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)									1.80%	2.50%	2.20%
Valuation allowance (as a percent)									0.30%	1.10%	3.30%
Foreign taxes (as a percent)									0.40%	0.60%	0.40%
Deferred tax adjustment (as a percent)									0.10%	1.10%	(0.90%)
Unrecognized tax benefit (as a percent)									(1.00%)	0.40%	(0.40%)
Other-net (as a percent)									1.00%	1.40%	2.70%
Effective income tax rate , Percent, Total									37.60%	42.10%	42.30%
Valuation allowance related to foreign subsidiary losses									$ 0.4	$ 1.8	$ 4.1
Deferred tax assets:
Benefit plans	78.1				79.1				78.1	79.1
Federal net operating losses	52.1				68.0				52.1	68.0
Insurance reserves	9.9				15.4				9.9	15.4
State net operating losses	7.9				7.8				7.9	7.8
Foreign losses	4.9				5.7				4.9	5.7
Alternative minimum tax credits	31.0				3.6				31.0	3.6
Allowance for doubtful accounts	1.3				2.4				1.3	2.4
Reserves	2.4								2.4
Other	3.5				1.9				3.5	1.9
Total deferred tax assets	191.1				183.9				191.1	183.9
Valuation allowance	(11.9)				(12.6)				(11.9)	(12.6)
Total Deferred tax assets, net of valuation allowance	179.2				171.3				179.2	171.3
Deferred tax liabilities:
Basis differences for property and equipment	339.0				300.8				339.0	300.8
Capital Construction Fund	115.8				95.6				115.8	95.6
Intangibles	53.8				53.0				53.8	53.0
Deferred revenue	9.5				11.8				9.5	11.8
Joint ventures and other investments	9.9				10.6				9.9	10.6
Reserves - Liabilities					10.0					10.0
Total deferred tax liabilities	528.0				481.8				528.0	481.8
Deferred tax liability, net , Total	$ 348.8				$ 310.5				$ 348.8	$ 310.5