PENSION AND OTHER POST-EMPLOYMENT BENEFITS - Components of Net Periodic Benefit Cost (Income) Pension and OBEB Plans (Details) - USD ($) $ in Millions	3 Months Ended							12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of the net periodic benefit cost
Settlement related expense	$ (10.5)	$ (9.8)	$ (0.5)	$ (0.2)	$ 0.5	$ 0.6	$ 4.6	$ 21.0	$ 5.7	$ 2.2
Net periodic benefit cost								(6.4)	(7.1)	(9.6)
Pension Benefits
Components of the net periodic benefit cost
Expected administrative expenses								1.7	0.9	0.7
Interest cost								9.0	10.2	9.4
Expected return on plan assets								(15.5)	(17.6)	(18.4)
Amortization of prior service cost								0.0	0.0	0.0
Recognized actuarial loss								0.8	1.0	0.7
Settlement related expense								21.0	5.7	2.2
Net periodic benefit cost								17.0	0.2	(5.4)
Other Benefits
Components of the net periodic benefit cost
Expected administrative expenses								0.0	0.0	0.0
Interest cost								0.1	0.1	0.2
Expected return on plan assets								0.0	0.0	0.0
Amortization of prior service cost								(0.7)	(0.7)	(1.5)
Recognized actuarial loss								0.1	(0.1)	0.0
Settlement related expense								(0.2)	0.0	0.0
Net periodic benefit cost								(0.7)	$ (0.7)	$ (1.3)
Union Pension Plan | Pension Benefits
Components of the net periodic benefit cost
Settlement related expense								14.5
Nonunion Pension Plan | Pension Benefits
Components of the net periodic benefit cost
Settlement related expense								$ 6.6