EMPLOYEE BENEFITS-COSTS AND CONTRIBUTIONS (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Components of net periodic benefit cost
Contributions and payments related to pension and OPEB plans	$ 5.3	$ 106.0
Pension Costs
Components of net periodic benefit cost
Service cost	12.0	11.7
Interest cost	28.3	31.2
Expected return on plan assets	(49.6)	(49.6)
Loss on plan settlement	0.4	0.0
Amortization of prior service cost (credit)	0.7	0.7
Amortization of net actuarial loss	23.1	21.9
Net periodic benefit (credit) cost	14.9	15.9
Contributions and payments related to pension and OPEB plans	3.7
Estimated future employer contributions for the remainder of the year	8.5
Other Postretirement Benefit Costs
Components of net periodic benefit cost
Service cost	6.2	6.3
Interest cost	7.5	8.5
Expected return on plan assets	(14.9)	(13.7)
Amortization of prior service cost (credit)	(3.8)	(2.8)
Amortization of net actuarial loss	0.3	1.5
Net periodic benefit (credit) cost	(4.7)	$ (0.2)
Contributions and payments related to pension and OPEB plans	1.6
Estimated future employer contributions for the remainder of the year	$ 7.9