BENEFIT PLANS - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1.0	$ 0.9	$ 2.0	$ 1.8
Interest cost	7.1	8.0	14.2	16.0
Expected return on plan assets	(9.3)	(9.6)	(18.6)	(19.2)
Amortization of prior service cost	0.2	0.2	0.4	0.4
Recognized actuarial loss	3.8	3.5	7.6	7.0
Total net periodic benefit cost	2.8	3.0	5.6	6.0
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.1	0.2	0.2
Interest cost	0.2	0.2	0.4	0.4
Expected return on plan assets	(0.3)	(0.3)	(0.6)	(0.6)
Amortization of prior service cost	(0.3)	(0.3)	(0.6)	(0.6)
Recognized actuarial loss	0.4	0.2	0.8	0.4
Total net periodic benefit cost	$ 0.1	$ (0.1)	$ 0.2	$ (0.2)