EMPLOYEE BENEFITS (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Components of net periodic benefit cost
Contributions and payments related to pension and OPEB plans			$ 66.2	$ 1.0
Pension Costs
Components of net periodic benefit cost
Service cost	$ 1.9	$ 2.1	4.5	4.6
Interest cost	6.6	7.0	13.4	13.8
Expected return on plan assets	(11.7)	(13.2)	(23.2)	(26.0)
Loss on plan settlement	0.0	3.2	0.0	3.2
Amortization of net actuarial loss	4.3	4.5	8.6	8.7
Net periodic benefit cost (credit)	1.1	3.6	3.3	4.3
Contributions and payments related to pension and OPEB plans			65.4
Estimated future employer contributions for the remainder of the year			0.3
Other Postretirement Benefit Costs
Components of net periodic benefit cost
Service cost	1.5	1.9	3.1	3.7
Interest cost	2.3	2.7	4.7	5.3
Expected return on plan assets	(4.1)	(4.0)	(8.2)	(8.0)
Amortization of prior service cost (credit)	(2.3)	(1.9)	(4.6)	(3.7)
Amortization of net actuarial loss	0.6	0.6	1.2	1.2
Net periodic benefit cost (credit)	$ (2.0)	$ (0.7)	(3.8)	$ (1.5)
Contributions and payments related to pension and OPEB plans			$ 0.8