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Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2020

Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash

The following table provides a reconciliation of cash, cash equivalents and restricted cash reported in the consolidated balance sheets that sum to the total of the same such amounts shown in the Consolidated Statements of Cash Flows (in thousands).

	Twelve Months Ended December 31, 2020				Twelve Months Ended December 31, 2019
	Beginning of period		End of period		Beginning of period		End of period
Cash and cash equivalents	$	189,680	$	326,028	$	134,758	$	189,680
Restricted cash		4,787		5,770		4,826		4,787
Total cash, cash equivalents and restricted cash shown in the statements of cash flows	$	194,467	$	331,798	$	139,584	$	194,467

Schedule of Estimated Useful Lives by Major Asset Category

Property and equipment are recorded at cost and depreciated over their estimated useful lives using the straight-line method. Leasehold improvements are amortized over the shorter of their estimated useful lives or the term of the lease by use of the straight-line method. Construction-in-process reflects amounts incurred for property, equipment or improvements that have not been placed in service. Maintenance and repair costs are expensed as incurred. When assets are retired or sold, the assets and accumulated depreciation are removed from the respective accounts and any gain or loss is recognized. Estimated useful lives by major asset category are as follows:

		Useful Lives
Machinery and equipment		5 to 7 years
Computers and software		3 years
Furniture and fixtures		10 years

Weighted-Average Assumptions Used to Estimate Fair Value of Employee Stock Options

The estimated fair value of each stock option and purchase right is then expensed over the requisite service period, which is generally the vesting period. The following weighted-average assumptions were used during these periods:

	Years Ended December 31,
	2020			2019			2018
Stock Options:
Expected volatility		63	%		62	%		71	%
Risk-free interest rate		1	%		2	%		3	%
Expected dividend yield		0	%		0	%		0	%
Expected life of options in years		5.5			5.7			5.7

Weighted-Average Assumptions Used to Estimate Fair Value of Employee Stock Purchase Plan Rights

	Years Ended December 31,
	2020			2019			2018
Employee Stock Purchase Plan:
Expected volatility	50%-76%			62%-86%			59%-79%
Risk-free interest rate	0.1%-0.2%			1.5%-2.4%			2.1%-2.8%
Expected dividend yield		0	%		0	%		0	%
Expected life in years	0.5-2.0			0.5-2.0			0.5-2.0

Summary of Stock-based Compensation Expense Included in Statements of Operations

The table below summarizes the total stock-based compensation expense included in the Company’s statements of operations for the periods presented (in thousands):

	Years Ended December 31,
	2020		2019		2018
Cost of product sales	$	2,632	$	2,936	$	3,863
Research and development		31,314		32,533		32,038
Sales, general and administrative		50,476		46,796		45,663
	$	84,422	$	82,265	$	81,564