EMPLOYEE BENEFIT PLANS - NET PERIODIC BENEFIT COST (Detail) (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	$ 178.7	$ 110.2
Prior service cost (credit)	1.8	2.4
Total	180.5	112.6
Estimated amounts that will be amortized as a component of net periodic benefit cost during 2014
Net actuarial losses	19.6
Prior service costs (credits)	0.2
Total 2015 - estimated amortization	19.8
Components of net periodic benefit cost (including amounts capitalized to the balance sheets)
Service cost	8.6	10.8	12.8
Interest cost	34.4	30.6	34.0
Expected return on plan assets	(64.1)	(57.2)	(55.4)
Loss on plan settlement	0.4	0	0
Amortization of transition obligation	0	0	0
Amortization of prior service cost (credit)	0.6	3.6	4.5
Amortization of net actuarial loss	15.0	24.0	14.9
Net periodic benefit cost	(5.1)	11.8	10.8
Other Benefits
Net regulatory assets
Net actuarial losses	41.0	11.5
Prior service cost (credit)	(78.3)	(12.0)
Total	(37.3)	(0.5)
Estimated amounts that will be amortized as a component of net periodic benefit cost during 2014
Net actuarial losses	4.2
Prior service costs (credits)	(9.3)
Total 2015 - estimated amortization	(5.1)
Components of net periodic benefit cost (including amounts capitalized to the balance sheets)
Service cost	7.7	10.6	8.5
Interest cost	11.5	13.4	15.1
Expected return on plan assets	(16.0)	(14.8)	(14.6)
Loss on plan settlement	0	0	0
Amortization of transition obligation	0	0	0.2
Amortization of prior service cost (credit)	(8.0)	(2.1)	(3.0)
Amortization of net actuarial loss	2.8	7.5	5.7
Net periodic benefit cost	$ (2.0)	$ 14.6	$ 11.9