EMPLOYEE BENEFITS-COSTS AND CONTRIBUTIONS (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Components of net periodic benefit cost
Contributions and payments related to pension and OPEB plans			$ 0.6	$ 66.5
Pension Costs
Components of net periodic benefit cost
Service cost	$ 2.6	$ 2.2	7.8	6.7
Interest cost	6.5	6.6	19.5	20.0
Expected return on plan assets	(12.1)	(11.6)	(36.2)	(34.8)
Amortization of net actuarial loss	5.3	4.4	15.8	13.0
Net periodic benefit (credit) cost	2.3	1.6	6.9	4.9
Contributions and payments related to pension and OPEB plans			0.5
Estimated future employer contributions for the remainder of the year	0.2		0.2
Other Postretirement Benefit Costs
Components of net periodic benefit cost
Service cost	1.5	1.5	4.6	4.6
Interest cost	2.0	2.3	6.1	7.0
Expected return on plan assets	(4.4)	(4.1)	(13.3)	(12.3)
Amortization of prior service (credit) cost	(2.8)	(2.3)	(8.5)	(6.9)
Amortization of net actuarial loss	0.7	0.7	1.9	1.9
Net periodic benefit (credit) cost	(3.0)	$ (1.9)	(9.2)	$ (5.7)
Contributions and payments related to pension and OPEB plans			0.1
Estimated future employer contributions for the remainder of the year	$ 0.1		$ 0.1