Stockholders' Equity (Details) $ / shares in Units, $ in Thousands				3 Months Ended					9 Months Ended		12 Months Ended
	Feb. 25, 2021 USD ($)	Mar. 12, 2020 USD ($)	Nov. 12, 2019 USD ($)	Sep. 30, 2021 USD ($) $ / shares shares	Jun. 30, 2021 shares	Mar. 31, 2021 USD ($) shares	Jun. 30, 2020 shares	Mar. 31, 2020 USD ($) shares	Sep. 30, 2021 USD ($) installment $ / shares shares	Sep. 30, 2020 USD ($) shares	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) shares	Jun. 04, 2020 shares	Feb. 28, 2014 shares
Class of Stock [Line Items]
Common stock, authorized (in shares)				350,000,000					350,000,000		350,000,000		175,000,000
Common stock, par value (in dollars per share) | $ / shares				$ 0.00001					$ 0.00001		$ 0.00001
Common stock, issued (in shares)				174,551,989					174,551,989		148,074,085
Common stock, outstanding (in shares)				174,551,989					174,551,989		148,074,085
Preferred stock, shares authorized (in shares)				25,000,000					25,000,000		25,000,000
Preferred stock, par value (in dollars per share) | $ / shares				$ 0.00001					$ 0.00001		$ 0.00001
Preferred stock, issued (in shares)				0					0		0
Preferred stock, outstanding (in shares)				0					0		0
Proceeds from the issuance of common stock, net of issuance costs | $									$ 82,799	$ 198,883
Inducement Award Program
Class of Stock [Line Items]
Number of shares granted									1,228,200
Number of shares outstanding				1,154,200					1,154,200
Share-based Payment Arrangement, Option
Class of Stock [Line Items]
Expiration period									10 years
Share-based Payment Arrangement, Option | Share-based Compensation Award, Tranche, First Anniversary of Grant Date
Class of Stock [Line Items]
Vesting percentage									100.00%
Share-based Payment Arrangement, Option | Share Based Compensation Award Tranche One
Class of Stock [Line Items]
Vesting period									1 year
Vesting percentage									25.00%
Share-based Payment Arrangement, Option | Share Based Compensation Award Tranche Two
Class of Stock [Line Items]
Vesting period									1 year
Number of equal quarterly installments | installment									12
Service-Based Restricted Stock Units | Share Based Compensation Award Tranche One
Class of Stock [Line Items]
Vesting period									1 year
Vesting percentage									50.00%
Service-Based Restricted Stock Units | Share Based Compensation Award Tranche Two
Class of Stock [Line Items]
Vesting period									6 months
Vesting percentage									25.00%
Award vesting grace period									1 year
Service-Based Restricted Stock Units | Share-based Compensation Award, Tranche, First Or Third Anniversary of Grant Date
Class of Stock [Line Items]
Vesting percentage									100.00%
Service-Based Restricted Stock Units | Share-Based Payment Arrangement, Tranche One, First Anniversary After Grant Date, Subjected To Individual Continued Service
Class of Stock [Line Items]
Vesting period									1 year
Vesting percentage									33.33%
Service-Based Restricted Stock Units | Share-Based Payment Arrangement, Tranche Two, Second Anniversary After Grant Date, Subjected To Individual Continued Service
Class of Stock [Line Items]
Vesting period									2 years
Vesting percentage									33.33%
Service-Based Restricted Stock Units | Share-Based Payment Arrangement, Tranche Three, Third Anniversary After Grant Date, Subjected To Individual Continued Service
Class of Stock [Line Items]
Vesting period									3 years
Vesting percentage									33.33%
Common Stock
Class of Stock [Line Items]
Common stock, outstanding (in shares)				174,551,989	169,651,423	158,520,089	143,129,409	130,251,440	174,551,989	143,328,652	148,074,085	121,674,568
Issuance of common stock, net of issuance costs (in shares)				4,730,466	10,446,160	9,228,017	12,650,000	7,973,967
Common Stock | At The Market Equity Offering Program Authorized February 2021
Class of Stock [Line Items]
Issuance of common stock, net of issuance costs (in shares)				4,730,466					19,180,365
Proceeds from the issuance of common stock, net of issuance costs | $				$ 16,100					$ 67,200
Minimum | Share-based Payment Arrangement, Option
Class of Stock [Line Items]
Vesting period									12 months
Minimum | Service-Based Restricted Stock Units | Share-based Compensation Award, Tranche, First Or Third Anniversary of Grant Date
Class of Stock [Line Items]
Vesting period									1 year
Maximum | At The Market Equity Offering Program Authorized February 2021
Class of Stock [Line Items]
Common stock sales agreement amount | $	$ 100,000
Maximum | Share-based Payment Arrangement, Option
Class of Stock [Line Items]
Vesting period									48 months
Maximum | Service-Based Restricted Stock Units | Share-based Compensation Award, Tranche, First Or Third Anniversary of Grant Date
Class of Stock [Line Items]
Vesting period									3 years
At The Market Equity Offering Program | Common Stock
Class of Stock [Line Items]
Issuance of common stock, net of issuance costs (in shares)						5,224,278		7,973,967			3,509,381	2,684,392
Proceeds from the issuance of common stock, net of issuance costs | $						$ 15,900		$ 56,700			$ 10,600	$ 16,800
At The Market Equity Offering Program | Maximum
Class of Stock [Line Items]
Common stock sales agreement amount | $		$ 65,000	$ 75,000
2014 Plans
Class of Stock [Line Items]
Common stock, reserved for future issuance (in shares)														1,785,000
Incremental rate at which the shares reserved for issuance increase									3.00%
2014 Plans | Share-based Payment Arrangement, Option | Employees
Class of Stock [Line Items]
Number of shares granted									1,997,200
2014 Plans | Share-based Payment Arrangement, Option | Director
Class of Stock [Line Items]
Number of shares granted									200,800
2014 Plans | Performance Shares
Class of Stock [Line Items]
Number of shares granted									99,558
2014 Plans | Service-Based Restricted Stock Units | Employees
Class of Stock [Line Items]
Number of shares granted (in shares)									3,396,012
2014 Plans | Service-Based Restricted Stock Units | Director
Class of Stock [Line Items]
Number of shares granted									82,200
2014 Plans | Performance-Based Restricted Stock Units
Class of Stock [Line Items]
Number of shares granted (in shares)									37,150
2014 Employee Stock Purchase Plan
Class of Stock [Line Items]
Common stock, reserved for future issuance (in shares)				5,173,141					5,173,141
Offering period									6 months
Purchase price at the end of offering period									85.00%
Number of shares issued									307,193