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
Liabilities held for sale	$ 0	$ 49.1
Long-term liabilities	274.6	200.8
Pension Benefits
Change in benefit obligation
Obligation at January 1	1,641.7	1,784.9
Service cost	24.8	30.2	46.0
Interest cost	76.2	71.2	78.0
Plan amendments	0	0
Divestitures - UPPCO	(100.4)	0
Actuarial loss (gain), net	166.1	(153.1)
Participant contributions	0	0
Benefit payments	(102.7)	(91.5)
Federal subsidy on benefits paid	0	0
Obligation at December 31	1,705.7	1,641.7	1,784.9
Change in fair value of plan assets
Fair value of plan assets at January 1	1,527.7	1,348.1
Actual return on plan assets	94.6	205.4
Employer contributions	98.8	65.7
Participant contributions	0	0
Divestitures - UPPCO	(122.8)	0
Benefit payments	(102.7)	(91.5)
Fair value of plan assets at December 31	1,495.6	1,527.7	1,348.1
Funded status at December 31	(210.1)	(114.0)
Amounts recognized on the entity's balance sheets related to the funded status of the benefit plans
Long-term assets	0	0
Current liabilities	9.1	8.9
Liabilities held for sale	0	6.9
Long-term liabilities	201.0	98.2
Total net liabilities	(210.1)	(114.0)
Accumulated benefit obligation	1,531.1	1,489.1
Information for pension plans with an accumulated benefit obligation in excess of plan assets
Fair value of plan assets	0
Projected benefit obligation	64.1	65.4
Accumulated benefit obligation	61.2	63.0
Other Benefits
Change in benefit obligation
Obligation at January 1	576.3	621.0
Service cost	21.0	24.9	20.8
Interest cost	23.5	24.8	28.5
Plan amendments	(90.4)	0.2
Divestitures - UPPCO	(22.3)	0
Actuarial loss (gain), net	33.1	(73.4)
Participant contributions	10.0	10.6
Benefit payments	(33.3)	(34.0)
Federal subsidy on benefits paid	2.1	2.2
Obligation at December 31	520.0	576.3	621.0
Change in fair value of plan assets
Fair value of plan assets at January 1	470.1	424.4
Actual return on plan assets	18.2	57.8
Employer contributions	10.0	11.3
Participant contributions	10.0	10.6
Divestitures - UPPCO	(27.3)	0
Benefit payments	(33.3)	(34.0)
Fair value of plan assets at December 31	447.7	470.1	424.4
Funded status at December 31	(72.3)	(106.2)
Amounts recognized on the entity's balance sheets related to the funded status of the benefit plans
Long-term assets	1.5	0
Current liabilities	0.2	0.2
Liabilities held for sale	0	3.4
Long-term liabilities	73.6	102.6
Total net liabilities	$ (72.3)	$ (106.2)