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 cost
Contributions and payments related to pension and OPEB plans			$ 8.3	$ 9.7
Pension Costs
Components of net periodic benefit cost
Service cost	$ 11.8	$ 11.8	23.1	23.8
Interest cost	30.2	28.7	60.8	57.0
Expected return on plan assets	(48.2)	(48.8)	(96.9)	(98.4)
Loss on plan settlement	1.0	0.3	1.8	0.7
Amortization of prior service (credit) cost	0.5	0.6	1.1	1.3
Amortization of net actuarial (gain) loss	18.7	23.9	37.7	47.0
Net periodic benefit (credit) cost	14.0	16.5	27.6	31.4
Contributions and payments related to pension and OPEB plans			6.9
Estimated future employer contributions for the remainder of the year	4.9		4.9
Other Postretirement Benefit Costs
Components of net periodic benefit cost
Service cost	3.8	5.6	8.2	11.8
Interest cost	6.3	7.4	12.8	14.9
Expected return on plan assets	(13.6)	(14.8)	(27.3)	(29.7)
Amortization of prior service (credit) cost	(3.8)	(3.9)	(7.7)	(7.7)
Amortization of net actuarial (gain) loss	(2.0)	0.2	(2.7)	0.5
Net periodic benefit (credit) cost	(9.3)	$ (5.5)	(16.7)	$ (10.2)
Contributions and payments related to pension and OPEB plans			1.4
Estimated future employer contributions for the remainder of the year	$ 5.3		$ 5.3