Stock Options And Employee Stock Purchase Plan (Tables)

6 Months Ended

Jun. 30, 2017

The following table represents stock option activity for the six months ended June 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	(36,183	)	29.05
Canceled	—		—
Outstanding at June 30, 2017	377,675		$	31.28	3.6	$	20,322,158
Exercisable at June 30, 2017	338,475		$	27.33	3.1	$	19,549,090
Vested and expected to vest at June 30, 2017	368,546		$	30.39	3.5	$	20,158,778

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



	June 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 six months ended June 30, 2017 is set forth below:



	Shares		Weighted-Average Grant-Date Fair Value
Nonvested at January 1, 2017	705,015		$	41.40
Granted	219,220		54.40
Vested	(176,341	)	45.17
Canceled	(1,850	)	87.68
Nonvested at June 30, 2017	746,044		$	44.22

Restricted stock unit award activity for the six months ended June 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	7,100
Vested	(9,530	)
Canceled	(6,680	)
Outstanding at June 30, 2017	42,840		1.9	$	3,645,256
Vested and expected to vest at June 30, 2017	32,409		1.7	$	2,757,659