EMPLOYEE BENEFITS-COSTS AND CONTRIBUTIONS (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Components of net periodic benefit costs
Contributions and payments related to pension and OPEB plans			$ 3.2	$ 3.8
Pension costs
Components of net periodic benefit costs
Service cost	$ 2.5	$ 3.3	6.3	6.6
Interest cost	11.1	10.5	22.6	21.1
Expected return on plan assets	(18.0)	(18.6)	(36.2)	(37.6)
Amortization of prior service (credit) cost	0.1	0.2	0.2	0.4
Amortization of net actuarial (gain) loss	6.8	9.6	14.0	19.0
Net periodic benefit (credit) cost	2.5	5.0	6.9	9.5
Contributions and payments related to pension and OPEB plans			2.5
Estimated future employer contributions for the remainder of the year	1.3		1.3
Other Postretirement Benefit Costs
Components of net periodic benefit costs
Service cost	1.0	1.7	2.2	3.5
Interest cost	2.4	2.7	4.8	5.5
Expected return on plan assets	(3.6)	(3.9)	(7.1)	(7.8)
Amortization of prior service (credit) cost	(0.5)	(0.5)	(1.0)	(1.1)
Amortization of net actuarial (gain) loss	(0.7)	0.0	(1.1)	0.0
Net periodic benefit (credit) cost	(1.4)	$ 0.0	(2.2)	$ 0.1
Contributions and payments related to pension and OPEB plans			0.7
Estimated future employer contributions for the remainder of the year	$ 2.9		$ 2.9