Stockholders' Equity (Tables)

6 Months Ended

Jun. 30, 2019

Stockholders' Equity and Stock-based Compensation Disclosure [Abstract]

Summary of Activity Related to Stock Option Plans

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	(1,200,996	)	1,200,996		339.01
Exercised	—		(1,236,285	)	36.67
Expired	—		(21	)	4.27
Balances as of June 30, 2019	7,498,945		20,443,968		$	107.46	5.62	$	5,320,942
Vested and exercisable as of June 30, 2019			20,443,968		$	107.46	5.62	$	5,320,942

Summary of Amounts Related to Option Exercises

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

	Three Months Ended				Six Months Ended
	June 30, 2019		June 30, 2018		June 30, 2019		June 30, 2018
	(in thousands)
Total intrinsic value of options exercised	$	212,932	$	246,795	$	393,774	$	524,705
Cash received from options exercised	21,896		26,936		44,868		83,271

Summary of Assumptions Used to Value Stock Option Grants Using Lattice-Binomial Model

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

	Three Months Ended						Six Months Ended
	June 30, 2019			June 30, 2018			June 30, 2019			June 30, 2018
Dividend yield	—		%	—		%	—		%	—		%
Expected volatility	39		%	42		%	39% - 41%			40% - 42%
Risk-free interest rate	2.50		%	2.90		%	2.50% - 2.74%			2.61% - 2.90%
Suboptimal exercise factor	3.16			2.93			3.07 - 3.16			2.80 - 2.93
Weighted-average fair value (per share)	$	179		$	161		$	171		$	142
Total stock-based compensation expense (in thousands)	$	103,848		$	81,232		$	205,048		$	149,627
Total income tax impact on provision (in thousands)	$	24,845		$	16,889		$	46,473		$	31,580