EMPLOYEE BENEFIT PLANS - NET PERIODIC BENEFIT COST (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Benefits
Net regulatory assets
Net actuarial losses	$ 43.2
Prior service costs (credits)	0.2
Total 2015 - estimated amortization	43.4
Components of net periodic benefit cost (including amounts capitalized to the balance sheets)
Service cost	24.8	30.2	46.0
Interest cost	76.2	71.2	78.0
Expected return on plan assets	(112.4)	(105.5)	(107.9)
Loss on plan settlement	0.9	0	0
Amortization of transition obligation	0	0	0
Amortization of prior service cost (credit)	0.6	4.0	5.0
Amortization of net actuarial loss	33.3	56.7	34.0
Net periodic benefit cost	23.4	56.6	55.1
Pension Benefits | Nonregulated segments
Accumulated other comprehensive loss (pre-tax)
Net actuarial loss	40.2	33.3
Prior service credits	0	0
Total	40.2	33.3
Pension Benefits | Utility segments
Net regulatory assets
Net actuarial loss	501.0	356.2
Prior service costs (credits)	1.8	2.4
Total	502.8	358.6
Other Benefits
Net regulatory assets
Net actuarial losses	4.4
Prior service costs (credits)	(10.3)
Total 2015 - estimated amortization	(5.9)
Components of net periodic benefit cost (including amounts capitalized to the balance sheets)
Service cost	21.0	24.9	20.8
Interest cost	23.5	24.8	28.5
Expected return on plan assets	(33.0)	(30.6)	(28.2)
Loss on plan settlement	0	0	0
Amortization of transition obligation	0	0	0.3
Amortization of prior service cost (credit)	(9.4)	(2.5)	(3.4)
Amortization of net actuarial loss	3.2	8.4	6.6
Net periodic benefit cost	5.3	25.0	24.6
Other Benefits | Nonregulated segments
Accumulated other comprehensive loss (pre-tax)
Net actuarial loss	0.2	0.7
Prior service credits	(0.1)	(0.2)
Total	0.1	0.5
Other Benefits | Utility segments
Net regulatory assets
Net actuarial loss	50.4	6.2
Prior service costs (credits)	(85.3)	(12.4)
Total	$ (34.9)	$ (6.2)