EMPLOYEE BENEFIT PLANS - CHANGE IN BENEFIT OBLIGATIONS AND PLAN ASSETS (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Amounts recognized on the entity's balance sheets related to the funded status of the benefit plans
Long-term assets	$ 128.9	$ 145.1
Long-term liabilities	37.6	76.4
Information for pension plans with an accumulated benefit obligation in excess of plan assets
Fair value of plan assets	0
Pension Benefits
Change in benefit obligation
Obligation at January 1	717.5	772.6
Service cost	8.6	10.8	12.8
Interest cost	34.4	30.6	34.0
Plan amendments	0	0
Transfer to affiliates	(12.1)	(6.6)
Actuarial loss (gain), net	73.0	(63.6)
Participant contributions	0	0
Benefit payments	(29.6)	(26.3)
Federal subsidy on benefits paid	0	0
Obligation at December 31	791.8	717.5	772.6
Change in fair value of plan assets
Fair value of plan assets at January 1	839.1	719.6
Actual return on plan assets	53.1	112.1
Employer contributions	46.9	40.3
Participant contributions	0	0
Benefit payments	(29.6)	(26.3)
Transfer to affiliates	(12.1)	(6.6)
Fair value of plan assets at December 31	897.4	839.1	719.6
Funded status at December 31	105.6	121.6
Amounts recognized on the entity's balance sheets related to the funded status of the benefit plans
Long-term assets	128.9	145.1
Current liabilities	1.5	3.1
Long-term liabilities	21.8	20.4
Total net assets (liabilities)	105.6	121.6
Accumulated benefit obligation	717.4	658.3
Information for pension plans with an accumulated benefit obligation in excess of plan assets
Projected benefit obligation	23.3	23.5
Accumulated benefit obligation	21.5	21.8
Other Benefits
Change in benefit obligation
Obligation at January 1	292.7	328.5
Service cost	7.7	10.6	8.5
Interest cost	11.5	13.4	15.1
Plan amendments	(74.4)	0.1
Transfer to affiliates	0	0
Actuarial loss (gain), net	24.0	(51.4)
Participant contributions	0.5	0.6
Benefit payments	(10.4)	(10.0)
Federal subsidy on benefits paid	0.9	0.9
Obligation at December 31	252.5	292.7	328.5
Change in fair value of plan assets
Fair value of plan assets at January 1	236.5	213.7
Actual return on plan assets	7.4	29.0
Employer contributions	2.6	3.2
Participant contributions	0.5	0.6
Benefit payments	(10.4)	(10.0)
Transfer to affiliates	0	0
Fair value of plan assets at December 31	236.6	236.5	213.7
Funded status at December 31	(15.9)	(56.2)
Amounts recognized on the entity's balance sheets related to the funded status of the benefit plans
Long-term assets	0	0
Current liabilities	0.1	0.2
Long-term liabilities	15.8	56.0
Total net assets (liabilities)	$ (15.9)	$ (56.2)