Stockholders' Equity (Tables)

9 Months Ended

Sep. 30, 2017

A summary of the activities related to the Company’s stock option plans is as follows:



			Options Outstanding
	Shares Available for Grant		Number of Shares		Weighted- Average Exercise Price (per share)		Weighted-Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value (in thousands)
Balances as of December 31, 2016	13,289,953		22,437,347		$	44.83
Granted	(1,988,266	)	1,988,266		151.82
Exercised	—		(2,676,918	)	27.53
Expired	—		(1,561	)	3.25
Balances as of September 30, 2017	11,301,687		21,747,134		$	56.74	6.04	$	2,709,943
Vested and exercisable as of September 30, 2017			21,747,134		$	56.74	6.04	$	2,709,943

A summary of the amounts related to option exercises, is as follows:



	Three Months Ended				Nine Months Ended
	September 30, 2017		September 30, 2016		September 30, 2017		September 30, 2016
	(in thousands)
Total intrinsic value of options exercised	$	142,664	$	35,443	$	351,488	$	104,168
Cash received from options exercised	34,669		3,819		73,673		11,587

The following table summarizes the assumptions used to value option grants using the lattice-binomial model and the valuation data:



	Three Months Ended						Nine Months Ended
	September 30, 2017			September 30, 2016			September 30, 2017			September 30, 2016
Dividend yield	—		%	—		%	—		%	—		%
Expected volatility	34		%	41		%	34% - 37%			41% - 50%
Risk-free interest rate	2.24		%	1.57		%	2.24% - 2.45%			1.57% - 2.04%
Suboptimal exercise factor	2.58			2.48			2.48 - 2.58			2.48
Weighted-average fair value (per share)	$	72.98		$	44.68		$	67.23		$	47.79
Total stock-based compensation expense (in thousands)	$	44,763		$	43,495		$	133,679		$	130,029
Total income tax impact on provision (in thousands)	$	14,428		$	16,294		$	43,606		$	48,828