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Stock Options And Employee Stock Purchase Plan (Tables)

3 Months Ended

Mar. 31, 2020

The following table represents stock option activity for the three months ended March 31, 2020:


	Number of Shares	Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Outstanding at January 1, 2020	518,341	$	65.77
Granted	—	—
Exercised	(41,530)	22.92
Canceled	—	—
Outstanding at March 31, 2020	476,811	$	69.51	6.9	$	2,619,831
Exercisable at March 31, 2020	172,211	$	59.94	5.4	$	2,585,331
Vested and expected to vest at March 31, 2020	382,175	$	68.14	6.7	$	2,619,410

The compensation expense related to the Purchase Plan has been estimated utilizing the following assumptions:


	March 31, 2020	March 31, 2019
Risk-free interest rate	1.57%	2.32%
Expected term (in years)	0.5	0.5
Dividend yield	—%	1.19%
Expected volatility	23.95%	22.17%
Weighted average volatility	23.95%	22.17%

The weighted-average fair values of market-based restricted stock awards granted have been estimated utilizing the following assumptions:


	March 31, 2020			March 31, 2019
Underlying stock price at valuation date	$	91.17		$	84.58
Expected volatility	27.0		%	28.3		%
Risk-free interest rate	0.71		%	2.53		%

Restricted stock award activity for the three months ended March 31, 2020 is set forth below:


	Shares	Weighted-Average Grant-Date Fair Value
Nonvested at January 1, 2020	1,105,059	$	64.76
Granted	1,268	98.63
Vested	(173,708)	65.97
Canceled	(4,681)	77.13
Nonvested at March 31, 2020	927,938	$	64.51

Restricted stock unit award activity for the three months ended March 31, 2020 is set forth below:


	Number of Shares	Weighted-Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Outstanding at January 1, 2020	20,874
Granted	179,361
Vested	(3,788)
Canceled	(1,920)
Outstanding at March 31, 2020	194,527	4.6	$	14,560,346
Vested and expected to vest at March 31, 2020	115,546	3.9	$	8,648,626