RESTRUCTURING - Rollforward of Restructuring Liability (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended	9 Months Ended	15 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2021	Sep. 30, 2021
Restructuring Reserve [Roll Forward]
Balance as of beginning of period	$ 18.3	$ 27.1	$ 44.5	$ 0.0	$ 44.5	$ 0.0
Accrued charges	11.6	15.9	23.9	94.1
Payments	(12.1)	(24.7)	(41.3)	(49.6)
Balance as of end of period	17.8	18.3	27.1	44.5	17.8	17.8
Total non-cash charges				24.9	16.6	41.5
Cumulative charges incurred through September 30, 2021	187.0				187.0	187.0
Employee Compensation
Restructuring Reserve [Roll Forward]
Balance as of beginning of period	16.5	24.1	44.5	0.0	44.5	0.0
Accrued charges	7.7	12.8	19.8	85.0
Payments	(9.2)	(20.4)	(40.2)	(40.5)
Balance as of end of period	15.0	16.5	24.1	44.5	15.0	15.0
Total non-cash charges				19.5	9.7	29.2
Cumulative charges incurred through September 30, 2021	154.5				154.5	154.5
Other Expenses
Restructuring Reserve [Roll Forward]
Balance as of beginning of period	1.8	3.0	0.0	0.0	0.0	0.0
Accrued charges	3.9	3.1	4.1	9.1
Payments	(2.9)	(4.3)	(1.1)	(9.1)
Balance as of end of period	2.8	$ 1.8	$ 3.0	0.0	2.8	2.8
Total non-cash charges				$ 5.4	6.9	12.3
Cumulative charges incurred through September 30, 2021	$ 32.5				$ 32.5	$ 32.5