Stockholders' Equity (Tables)

6 Months Ended

Jun. 30, 2018

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, 2017	10,739,915		21,647,350		$	61.13
Granted	(1,056,538	)	1,056,538		282.22
Exercised	—		(2,064,819	)	39.92
Expired	—		(1,820	)	4.72
Balances as of June 30, 2018	9,683,377		20,637,249		$	74.58	5.85	$	6,538,997
Vested and exercisable as of June 30, 2018			20,637,249		$	74.58	5.85	$	6,538,997

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, 2018		June 30, 2017		June 30, 2018		June 30, 2017
	(in thousands)
Total intrinsic value of options exercised	$	246,795	$	101,727	$	524,705	$	208,824
Cash received from options exercised	26,936		14,826		83,271		39,004

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, 2018			June 30, 2017			June 30, 2018			June 30, 2017
Dividend yield	—		%	—		%	—		%	—		%
Expected volatility	42		%	34		%	40% - 42%			34% - 37%
Risk-free interest rate	2.90		%	2.37		%	2.61 % - 2.90%			2.37%-2.45%
Suboptimal exercise factor	2.93			2.51			2.80 - 2.93			2.48 - 2.51
Weighted-average fair value (per share)	$	161.39		$	67.21		$	141.62		$	64.67
Total stock-based compensation expense (in thousands)	$	81,232		$	44,028		$	149,627		$	88,916
Total income tax impact on provision (in thousands)	$	16,889		$	14,477		$	31,580		$	29,178