PENSION AND POST-RETIREMENT BENEFITS - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($) plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 1.3	$ 1.5	$ 3.6	$ 4.2		$ 4.0
Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Aggregate employer contributions during 2021	21.0		21.0
Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Aggregate employer contributions during 2021	24.0		24.0
Forecast
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost					$ 5.0
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	1.1	1.3	3.0	3.6
Employer contributions	3.1		19.9
Pension Plans | Qualified Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions, expected deferral in 2020, CARES Act						$ 11.8
Number of qualified defined benefit plans | plan						2
Other Post-Retirement Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	0.2	$ 0.2	0.6	$ 0.6
Employer contributions	$ 0.2		$ 0.6