Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2021

Stock-based Compensation Expense

The stock-based compensation related to our stock-based awards and employee stock purchase plan for the year ended December 31, 2021, 2020 and 2019 is as follows (in thousands):

	Year Ended December 31,
	2021		2020		2019
Cost of net revenues	$	5,633	$	4,719	$	5,154
Selling, general and administrative	90,659		78,500		69,817
Research and development	18,044		15,208		13,213
Total stock-based compensation	$	114,336	$	98,427	$	88,184

Fair value assumptions using the Monte Carlo simulation

The weighted average assumptions used in the Monte Carlo simulation were as follows: 

	Year Ended December 31,
	2021			2020			2019
Expected term (in years)	3.0			3.0			3.0
Expected volatility	56.3		%	44.4		%	37.3		%
Risk-free interest rate	0.2		%	1.4		%	2.5		%
Expected dividends	—			—			—
Weighted average fair value per share at grant date	$	1,102.09		$	392.67		$	392.03

Schedule of ESPP share activity

The following table summarizes the ESPP shares issued:

	Year Ended December 31,
	2021		2020		2019
Number of shares issued (in thousands)	131		116		130
Weighted average price	$	195.44	$	175.69	$	136.73

Weighted Average Assumptions Used for the Fair Value of Options Component of Purchase Plan Granted Estimated at Grant Date

The fair value of the option component of the 2010 Purchase Plan shares was estimated at the grant date using the Black-Scholes option pricing model with the following weighted average assumptions:

	Year Ended December 31,
	2021			2020			2019
Expected term (in years)	1.1			1.0			1.4
Expected volatility	52.7		%	55.0		%	50.0		%
Risk-free interest rate	0.1		%	0.9		%	2.2		%
Expected dividends	—			—			—
Weighted average fair value at grant date	$	246.84		$	96.94		$	86.02

Restricted Stock Units (RSUs)

Summary of Nonvested Shares

A summary for the year ended December 31, 2021 is as follows:

	Number of Shares Underlying RSUs (in thousands)	Weighted Average Grant Date Fair Value		Weighted Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value (in thousands)
Unvested as of December 31, 2020	632	$	243.55
Granted	166	600.10
Vested and released	(265)	216.73
Forfeited	(41)	350.75
Unvested as of December 31, 2021	492	$	369.17	1.1	$	323,239

Market Performance Based Restricted Stock Units

Summary of Nonvested Shares

The following table summarizes the MSU performance for the year ended December 31, 2021:

	Number of Shares Underlying MSUs (in thousands)	Weighted Average Grant Date Fair Value		Weighted Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value (in thousands)
Unvested as of December 31, 2020	227	$	430.50
Granted 1	177	658.02
Vested and released	(230)	513.73
Unvested as of December 31, 2021	174	$	551.57	1.0	$	114,414