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Stock Options And Employee Stock Purchase Plan (Tables)

6 Months Ended

Jun. 30, 2020

The following table represents stock option activity for the six months ended June 30, 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 June 30, 2020	476,811	$	69.51	6.6	$	1,820,971
Exercisable at June 30, 2020	176,811	$	60.13	5.2	$	1,820,971
Vested and expected to vest at June 30, 2020	388,971	$	68.26	6.4	$	1,820,971

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


	June 30, 2020	June 30, 2019
Risk-free interest rate	0.1%	2.4%
Expected term (in years)	0.5	0.5
Dividend yield	—%	1.1%
Expected volatility	24.0%	26.8%
Weighted average volatility	24.0%	26.8%

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


	June 30, 2020			June 30, 2019
Underlying stock price at valuation date	$	91.17		$	84.58
Expected volatility	27.0		%	28.3		%
Risk-free interest rate	0.7		%	2.5		%

Restricted stock award activity for the six months ended June 30, 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	(242,783)	69.78
Canceled	(4,681)	77.13
Nonvested at June 30, 2020	858,863	$	63.32

Restricted stock unit award activity for the six months ended June 30, 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	197,078
Vested	(5,218)
Canceled	(1,920)
Outstanding at June 30, 2020	210,814	4.1	$	13,325,553
Vested and expected to vest at June 30, 2020	133,910	3.4	$	8,464,473