Stock Options And Employee Stock Purchase Plan (Tables)

3 Months Ended

Mar. 31, 2018

The following table represents stock option activity for the three months ended March 31, 2018:



	Number of Shares		Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Outstanding at January 1, 2018	375,675		$	31.30
Granted	400,000		75.03
Exercised	(27,679	)	21.45
Canceled	—		—
Outstanding at March 31, 2018	747,996		$	55.05	6.3	$	17,856,319
Exercisable at March 31, 2018	334,196		$	30.62	2.2	$	16,140,653
Vested and expected to vest at March 31, 2018	601,462		$	50.20	5.5	$	17,274,251

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



	March 31, 2018
Underlying stock price at valuation date	$	75.03
Expected volatility	28.4		%
Risk-free interest rate	2.34		%

Restricted stock award activity for the three months ended March 31, 2018 is set forth below:



	Shares		Weighted-Average Grant-Date Fair Value
Nonvested at January 1, 2018	605,566		$	51.57
Granted	612,547		26.09
Vested	(15,032	)	62.75
Canceled	(12,500	)	83.26
Nonvested at March 31, 2018	1,190,581		$	37.99

Restricted stock unit award activity for the three months ended March 31, 2018 is set forth below:



	Number of Shares		Weighted-Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Outstanding at January 1, 2018	38,400
Granted	12,614
Vested	(3,020	)
Canceled	—
Outstanding at March 31, 2018	47,994		2.0	$	3,787,686
Vested and expected to vest at March 31, 2018	38,597		1.8	$	3,046,076