EMPLOYEE BENEFITS-COSTS AND CONTRIBUTIONS (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Components of net periodic benefit cost (credit)
Contributions and payments related to pension and OPEB plans			$ 7.6	$ 6.6
Pension Benefits
Components of net periodic benefit cost (credit)
Service cost	$ 13.6	$ 11.7	27.5	24.8
Interest cost	21.7	26.1	43.6	52.2
Expected return on plan assets	(50.1)	(47.7)	(100.7)	(95.6)
Loss on plan settlement	1.9	10.0	2.0	10.3
Amortization of prior service (credit) cost	0.4	0.4	0.8	0.8
Amortization of net actuarial (gain) loss	28.2	25.5	55.6	49.7
Net periodic benefit (credit) cost	15.7	26.0	28.8	42.2
Contributions and payments related to pension and OPEB plans			6.6
Estimated future employer contributions for the remainder of the year	5.2		5.2
Other Postretirement Benefits
Components of net periodic benefit cost (credit)
Service cost	3.6	3.5	7.8	7.6
Interest cost	3.6	4.6	7.2	9.3
Expected return on plan assets	(16.6)	(15.1)	(33.0)	(30.2)
Amortization of prior service (credit) cost	(3.9)	(3.8)	(7.9)	(7.5)
Amortization of net actuarial (gain) loss	(6.5)	(5.8)	(12.2)	(11.2)
Net periodic benefit (credit) cost	(19.8)	$ (16.6)	(38.1)	$ (32.0)
Contributions and payments related to pension and OPEB plans			1.0
Estimated future employer contributions for the remainder of the year	$ 1.2		$ 1.2