STOCK-BASED COMPENSATION (Tables)

3 Months Ended

Mar. 31, 2020

Share-based Payment Arrangement [Abstract]

Schedule of valuation assumptions

​	Three Months Ended March 31,
​	2020	2019
Option Plan Shares	​	​
Risk-free interest rates	1.26% - 1.47%	2.54% - 2.59%
Expected term (in years)	6.15	6.28
Expected volatility	65.67% - 65.71%	64.95% - 64.99%
Dividend yield	—%	—%
Weighted average fair value per share of options granted during the period	$58.86	$57.11

Summary of stock option activity under the Stock Plans

Options	Shares	Weighted Average Exercise Price		Weighted Average Remaining Contractual Term(Years)	Aggregate Intrinsic Value(1)
(Aggregate intrinsic value in thousands)	​	​		​	​
Outstanding, January 1, 2020	2,700,293	$	34.01	2.9
Granted	309,143	97.66		​
Exercised	(160,286)	26.82		​
Forfeited	(8,558)	71.52		​
Outstanding, March 31, 2020	2,840,592	$	41.23	6.5	$	74,394
Exercisable, March 31, 2020	1,881,310	$	27.95	5.5	$	63,816

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(1)The total intrinsic value of options exercised during the three months ended March 31, 2020 and 2019 was $10.2 million and $16.1 million, respectively, determined as of the date of exercise.

Summary of restricted stock and restricted stock unit activity under the Stock Plans

A summary of restricted stock and restricted stock unit activity under the Stock Plans during the three months ended March 31, 2020 is as follows:

​	Restricted Shares and RSUs	Weighted Average Grant Date Fair Value
Outstanding, January 1, 2020	4,384,005	$	63.30
Granted	1,701,650	93.26
Released	(1,193,845)	45.76
Forfeited	(68,540)	74.29
Outstanding, March 31, 2020	4,823,270	$	78.06