Stockholders' Equity (Tables)

9 Months Ended

Sep. 30, 2018

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, 2017	10,739,915		21,647,350		$	61.13
Granted	(1,507,700	)	1,507,700		306.98
Exercised	—		(2,692,309	)	41.75
Expired	—		(2,051	)	4.64
Balances as of September 30, 2018	9,232,215		20,460,690		$	81.80	5.76	$	5,984,520
Vested and exercisable as of September 30, 2018			20,460,690		$	81.80	5.76	$	5,984,520

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



	Three Months Ended				Nine Months Ended
	September 30, 2018		September 30, 2017		September 30, 2018		September 30, 2017
	(in thousands)
Total intrinsic value of options exercised	$	193,411	$	142,664	$	718,116	$	351,488
Cash received from options exercised	29,781		34,669		113,052		73,673

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, 2018			September 30, 2017			September 30, 2018			September 30, 2017
Dividend yield	—		%	—		%	—		%	—		%
Expected volatility	40		%	34		%	40% - 42%			34% - 37%
Risk-free interest rate	2.90		%	2.24		%	2.61% - 2.90%			2.24%-2.45%
Suboptimal exercise factor	2.97			2.58			2.80 - 2.97			2.48 - 2.58
Weighted-average fair value (per share)	$	182.45		$	72.98		$	153.84		$	67.23
Total stock-based compensation expense (in thousands)	$	82,316		$	44,763		$	231,943		$	133,679
Total income tax impact on provision (in thousands)	$	18,036		$	14,428		$	49,617		$	43,606