INCOME TAXES - SUMMARY (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Current:
Federal									$ 22.6	$ 45.5	$ (24.3)
State									2.9	3.7	(1.0)
Total									25.5	49.2	(25.3)
Deferred:									49.3	2.7	57.5
Total income taxes expense	$ 14.4	$ 25.6	$ 19.2	$ 15.6	$ 17.3	$ 19.1	$ 13.1	$ 2.4	$ 74.8	$ 51.9	$ 32.2
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)									2.50%	2.20%	2.90%
Valuation allowance (as a percent)									1.10%	3.30%
Foreign taxes (as a percent)									0.60%	0.40%
Deferred tax adjustment (as a percent)									1.10%	(0.90%)
Unrecognized tax benefit (as a percent)									0.40%	(0.40%)	(2.10%)
Other-net (as a percent)									1.40%	2.70%	1.70%
Effective income tax rate , Percent, Total									42.10%	42.30%	37.50%
Valuation allowance related to foreign subsidiary losses									$ 1.8	$ 4.1
Valuation allowance recorded											$ 0.0
Deferred tax assets:
Benefit plans	79.1				63.6				79.1	63.6
Federal net operating losses	68.0								68.0
Insurance reserves	15.4				11.4				15.4	11.4
State net operating losses	7.8								7.8
Foreign losses	5.7				4.1				5.7	4.1
Alternative minimum tax credits	3.6				1.5				3.6	1.5
Allowance for doubtful accounts	2.4				1.4				2.4	1.4
Other	1.9				0.8				1.9	0.8
Total deferred tax assets	183.9				82.8				183.9	82.8
Valuation allowance	(12.6)				(4.1)				(12.6)	(4.1)
Total Deferred tax assets, net of valuation allowance	171.3				78.7				171.3	78.7
Deferred tax liabilities:
Basis differences for property and equipment	300.8				252.7				300.8	252.7
Capital Construction Fund	95.6				106.9				95.6	106.9
Intangibles	53.0				3.8				53.0	3.8
Deferred revenue	11.8				10.1				11.8	10.1
Joint ventures and other investments	10.6				7.7				10.6	7.7
Reserves - Assets					(2.1)					(2.1)
Reserves - Liabilities	10.0								10.0
Total deferred tax liabilities	481.8				379.1				481.8	379.1
Deferred tax liability, net , Total	$ 310.5				300.4				$ 310.5	300.4
Deferred income taxes, net current					$ 8.0					$ 8.0