Stockholders' Equity (Tables)

9 Months Ended

Sep. 30, 2019

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, 2018	8,699,941		20,479,278		$	89.61
Granted	(1,850,795	)	1,850,795		333.73
Exercised	—		(1,652,692	)	34.29
Expired	—		(175	)	6.17
Balances as of September 30, 2019	6,849,146		20,677,206		$	115.88	5.57	$	3,361,640
Vested and exercisable as of September 30, 2019			20,677,206		$	115.88	5.57	$	3,361,640

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



	Three Months Ended				Nine Months Ended
	September 30, 2019		September 30, 2018		September 30, 2019		September 30, 2018
	(in thousands)
Total intrinsic value of options exercised	$	119,439	$	193,411	$	513,213	$	718,116
Cash received from options exercised	11,989		29,781		56,857		113,052

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, 2019			September 30, 2018			September 30, 2019			September 30, 2018
Dividend yield	—		%	—		%	—		%	—		%
Expected volatility	38		%	40		%	38% - 41%			40% - 42%
Risk-free interest rate	1.92		%	2.90		%	1.92% - 2.74%			2.61% - 2.90%
Suboptimal exercise factor	3.19			2.97			3.07 - 3.19			2.80 - 2.97
Weighted-average fair value (per share)	$	154		$	182		$	165		$	154
Total stock-based compensation expense (in thousands)	$	100,262		$	82,316		$	305,310		$	231,943
Total income tax impact on provision (in thousands)	$	22,679		$	18,036		$	69,152		$	49,617