STOCK-BASED COMPENSATION PLANS - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Options
Outstanding, beginning of period (in shares)	588,024	466,677	983,512
Granted (in shares)	136,506	134,450	82,580
Exercised (in shares)	(86,145)	(4,868)	(582,303)
Canceled (in shares)	(85,716)	(8,235)	(17,112)
Outstanding, end of period (in shares)	552,669	588,024	466,677	983,512
Exercisable, end of period (in shares)	336,114
Weighted Average Exercise Price
Outstanding, beginning of period (in dollars per share)	$ 63.36	$ 61.04	$ 32.52
Granted (in dollars per share)	77.75	70.44	77.54
Exercised (in dollars per share)	35.47	22.45	28.29
Canceled (in dollars per share)	73.73	73.00	69.76
Outstanding, end of period (in dollars per share)	69.82	$ 63.36	$ 61.04	$ 32.52
Exercisable, end of period (in dollars per share)	$ 58.52
Weighted Average Remaining Contractual Life
Outstanding (in years)	6 years 1 month 6 days	5 years 8 months 12 days	5 years 8 months 12 days	4 years
Exercisable, end of period (in years)	5 years
Aggregate Intrinsic Value
Outstanding, beginning of period	$ 8.5	$ 4.3	$ 40.1
Granted	0.0	1.0	0.0
Exercised	0.0	(0.3)	(24.4)
Canceled	0.0	0.0	0.0
Outstanding, end of period	4.2	$ 8.5	$ 4.3	$ 40.1
Exercisable, end of period	$ 3.9

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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- Definition

The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.