Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2018

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 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, 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
Granted	(2,039,974	)	2,039,974		311.66
Exercised	—		(3,205,911	)	38.66
Expired	—		(2,135	)	4.60
Balances as of December 31, 2018	8,699,941		20,479,278		$	89.61	5.71	$	3,748,339
Vested and exercisable at December 31, 2018			20,479,278		$	89.61	5.71	$	3,748,339

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,
	2018			2017			2016
Dividend yield	—		%	—		%	—		%
Expected volatility	40% - 42%			34% - 37%			40% - 50%
Risk-free interest rate	2.61% - 3.09%			2.24% - 2.45%			1.57% - 2.04%
Suboptimal exercise factor	2.80 - 3.01			2.48 - 2.63			2.48
Valuation data:
Weighted-average fair value (per share)	$	157.19		$	71.45		$	48.85
Total stock-based compensation expense (in thousands)	320,657			182,209			173,675
Total income tax impact on provision (in thousands)	67,575			61,842			65,173