EMPLOYEE BENEFITS-COSTS AND CONTRIBUTIONS (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Components of net periodic benefit cost
Contributions and payments related to pension and OPEB plans			$ 6.6	$ 8.3
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 11.7	$ 11.8	24.8	23.1
Interest cost	26.1	30.2	52.2	60.8
Expected return on plan assets	(47.7)	(48.2)	(95.6)	(96.9)
Loss on plan settlement	10.0	1.0	10.3	1.8
Amortization of prior service (credit) cost	0.4	0.5	0.8	1.1
Amortization of net actuarial (gain) loss	25.5	18.7	49.7	37.7
Net periodic benefit (credit) cost	26.0	14.0	42.2	27.6
Contributions and payments related to pension and OPEB plans			6.1
Estimated future employer contributions for the remainder of the year	5.4		5.4
Other Postretirement Benefits
Components of net periodic benefit cost
Service cost	3.5	3.8	7.6	8.2
Interest cost	4.6	6.3	9.3	12.8
Expected return on plan assets	(15.1)	(13.6)	(30.2)	(27.3)
Amortization of prior service (credit) cost	(3.8)	(3.8)	(7.5)	(7.7)
Amortization of net actuarial (gain) loss	(5.8)	(2.0)	(11.2)	(2.7)
Net periodic benefit (credit) cost	(16.6)	$ (9.3)	(32.0)	$ (16.7)
Contributions and payments related to pension and OPEB plans			0.5
Estimated future employer contributions for the remainder of the year	$ 0.5		$ 0.5