Stock Options And Employee Stock Purchase Plan (Tables)

9 Months Ended

Sep. 30, 2017

The following table represents stock option activity for the nine months ended September 30, 2017:



	Number of Shares		Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Outstanding at January 1, 2017	413,858		$	31.09
Granted	—		—
Exercised	(38,183	)	29.03
Canceled	—		—
Outstanding at September 30, 2017	375,675		$	31.30	3.3	$	15,997,701
Exercisable at September 30, 2017	338,475		$	27.33	2.9	$	15,754,785
Vested and expected to vest at September 30, 2017	367,885		$	30.53	3.3	$	15,946,834

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



	September 30, 2017
Underlying stock price at valuation date	$	91.17
Expected volatility	29.0		%
Risk-free interest rate	2.17		%

Restricted stock award activity for the nine months ended September 30, 2017 is set forth below:



	Shares		Weighted-Average Grant-Date Fair Value
Nonvested at January 1, 2017	705,015		$	41.40
Granted	287,920		61.29
Vested	(216,410	)	45.27
Canceled	(1,850	)	87.68
Nonvested at September 30, 2017	774,675		$	47.60

Restricted stock unit award activity for the nine months ended September 30, 2017 is set forth below:



	Number of Shares		Weighted-Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Outstanding at January 1, 2017	51,950
Granted	11,100
Vested	(13,570	)
Canceled	(6,680	)
Outstanding at September 30, 2017	42,800		1.9	$	3,162,064
Vested and expected to vest at September 30, 2017	32,427		1.7	$	2,395,693