Stockholders' Equity (Tables)

6 Months Ended

Jun. 30, 2021

Equity [Abstract]

Schedule of Common Shares Reserved for Future Issuance

The following shares of the Company’s common stock were reserved for future issuance as of June 30, 2021 (in thousands):

Shares reserved for issuance upon conversion of Convertible Senior Notes	2,554
Shares underlying outstanding warrants	251
Shares underlying outstanding stock options	5,691
Shares underlying unvested restricted stock units	453
Shares authorized for issuance pursuant to awards granted under the ESPP	1,289
Shares authorized for future equity award grants	1,440
	11,678

Schedule of Stockholders' Equity Common Stock Warrants

Warrants outstanding to purchase shares of the Company's common stock as of June 30, 2021 were as follows:

Issue Date	Exercise Price Per Share		Warrants Outstanding	Expiration Date
October 2017	$	3.50	29,700	October 2022
March 2017	$	23.50	193,788	March 2027
August 2011 - August 2012	$	73.73	27,704	August 2021 - August 2022
			251,192

Schedule of Share-based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions

The assumptions used in the Black-Scholes option-pricing model for common stock options were as follows:

	Stock Options
	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Weighted average grant date fair value (per share)	$54.92		$52.50		$56.89		$50.04
Risk-free interest rate	1.1	%	0.4	%	1.0	%	0.6	%
Expected dividend yield	0.0	%	0.0	%	0.0	%	0.0	%
Expected volatility	74.9	%	75.0	%	75.1	%	74.3	%
Expected term (in years)	6.1		6.0		6.1		6.0

The weighted average grant date fair value of RSUs granted were as follows:

	Restricted Stock Units
	Three Months Ended June 30,		Six Months Ended June 30,
	2021	2020	2021	2020
Weighted average grant date fair value (per share)	$82.31	$82.33	$82.41	$82.33

The Company records stock-based compensation expense associated with the ESPP using the Black-Scholes option-pricing model. Valuations are performed on the grant date at the beginning of the purchase period, which generally occurs in May and November of each year. The assumptions used in the Black-Scholes option-pricing model for the ESPP were as follows:

	ESPP
	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Weighted average grant date fair value (per share)	$29.24		$36.18		$29.24		$36.18
Risk-free interest rate	0.1	%	0.2	%	0.1	%	0.2	%
Expected dividend yield	0.0	%	0.0	%	0.0	%	0.0	%
Expected volatility	47.5	%	65.7	%	47.5	%	65.7	%
Expected term (in years)	1.3		1.3		1.3		1.3

Schedule for Allocation of Stock-Based Compensation Expense

The following table summarizes the allocation of stock-based compensation expense included in the condensed consolidated statements of operations for all stock-based compensation arrangements (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Cost of sales	$	1,591	$	2,162	$	3,067	$	4,326
Selling, general & administrative	10,881		11,707		20,292		23,210
Research and development	2,505		2,552		4,565		4,750
Total stock-based compensation expense	$	14,977	$	16,421	$	27,924	$	32,286