EMPLOYEE BENEFITS-COSTS AND CONTRIBUTIONS (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Components of net periodic benefit cost
Contributions and payments related to pension and OPEB plans			$ 9.7	$ 111.5
Pension Costs
Components of net periodic benefit cost
Service cost	$ 11.8	$ 10.4	23.8	22.1
Interest cost	28.7	30.2	57.0	61.4
Expected return on plan assets	(48.8)	(48.5)	(98.4)	(98.1)
Loss on plan settlement	0.3	5.3	0.7	5.3
Amortization of prior service cost (credit)	0.6	0.8	1.3	1.5
Amortization of net actuarial loss	23.9	21.1	47.0	43.0
Net periodic benefit (credit) cost	16.5	19.3	31.4	35.2
Contributions and payments related to pension and OPEB plans			7.0
Estimated future employer contributions for the remainder of the year	65.6		65.6
Other Postretirement Benefit Costs
Components of net periodic benefit cost
Service cost	5.6	5.6	11.8	11.9
Interest cost	7.4	8.4	14.9	16.9
Expected return on plan assets	(14.8)	(13.6)	(29.7)	(27.3)
Amortization of prior service cost (credit)	(3.9)	(2.8)	(7.7)	(5.6)
Amortization of net actuarial loss	0.2	0.1	0.5	1.6
Net periodic benefit (credit) cost	(5.5)	$ (2.3)	(10.2)	$ (2.5)
Contributions and payments related to pension and OPEB plans			2.7
Estimated future employer contributions for the remainder of the year	$ 5.4		$ 5.4