Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2017

Stockholders' Equity Note [Abstract]

Summary of Activity Related to Stock Option Plans

A summary of the activities related to the Company’s stock option plans, as adjusted for the Stock Split, is as follows:

	Shares Available for Grant		Options Outstanding				Weighted- Average Remaining Contractual Term (in Years)	Aggregate Intrinsic Value (in Thousands)
			Number of Shares		Weighted- Average Exercise Price (per Share)
Balances as of December 31, 2014	20,025,208		22,845,417		$	21.65
Granted	(3,179,892	)	3,179,892		82.67
Exercised	—		(5,029,553	)	15.38
Balances as of December 31, 2015	16,845,316		20,995,756		$	32.39
Granted	(3,555,363	)	3,555,363		102.03
Exercised	—		(2,113,772	)	17.48
Balances as of December 31, 2016	13,289,953		22,437,347		$	44.83
Granted	(2,550,038	)	2,550,038		159.56
Exercised	—		(3,338,474	)	26.79
Expired	—		(1,561	)	3.25
Balances as of December 31, 2017	10,739,915		21,647,350		$	61.13	5.97	$	2,833,198
Vested and exercisable at December 31, 2017			21,647,350		$	61.13	5.97	$	2,833,198

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:

	Year Ended December 31,
	2017			2016			2015
Dividend yield	—		%	—		%	—		%
Expected volatility	34% - 37%			40% - 50%			36% - 53%
Risk-free interest rate	2.24% - 2.45%			1.57% - 2.04%			2.03% - 2.29%
Suboptimal exercise factor	2.48 - 2.63			2.48			2.47 - 2.48
Valuation data:
Weighted-average fair value (per share)	$	71.45		$	48.85		$	39.22
Total stock-based compensation expense (in thousands)	182,209			173,675			124,725
Total income tax impact on provision (in thousands)	61,842			65,173			47,125