Stockholders' Equity - Additional Information (Details)					1 Months Ended	3 Months Ended					9 Months Ended		12 Months Ended
	Mar. 12, 2020 USD ($)	Feb. 28, 2020 shares	Nov. 12, 2019 USD ($)	Dec. 12, 2018 shares	May 31, 2020 USD ($) $ / shares shares	Sep. 30, 2020 USD ($) $ / shares shares	Jun. 30, 2020 shares	Mar. 31, 2020 USD ($) shares	Sep. 30, 2019 USD ($) shares	Jun. 30, 2019 shares	Sep. 30, 2020 USD ($) installment $ / shares shares	Sep. 30, 2019 USD ($) shares	Dec. 31, 2019 USD ($) $ / shares shares	Jun. 04, 2020 shares	Mar. 31, 2019 shares	Dec. 31, 2018 shares	Feb. 28, 2014 shares
Class of Stock [Line Items]
Common stock, shares authorized (in shares)						350,000,000					350,000,000		175,000,000	175,000,000
Common stock, par value (in dollars per share) | $ / shares						$ 0.00001					$ 0.00001		$ 0.00001
Common stock, shares issued (in shares)						143,328,652					143,328,652		121,674,568
Common stock, shares outstanding (in shares)						143,328,652					143,328,652		121,674,568
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, shares issued (in shares)						0					0		0
Preferred stock, shares outstanding (in shares)						0					0		0
Proceeds from the issuance of common stock, net of issuance costs | $											$ 198,883,000	$ 9,035,000
Common stock, shares reserved for future issuance (in shares)						24,800,299					24,800,299		21,403,890
Stock-based compensation expense | $						$ 6,592,000			$ 2,613,000		$ 18,372,000	6,991,000
Follow-On Public Offering
Class of Stock [Line Items]
Number of shares sold in offering (in shares)					12,650,000
Offering price per share (in dollars per share) | $ / shares					$ 12.00
Aggregate net proceeds from offering | $					$ 142,400,000
Over-Allotment Option
Class of Stock [Line Items]
Number of shares sold in offering (in shares)					1,650,000
Inducement Award Program
Class of Stock [Line Items]
Number of shares granted (in shares)											948,250
Number of shares outstanding (in shares)						907,250					907,250
Inducement Award Program | Employees
Class of Stock [Line Items]
Number of shares granted (in shares)											948,250
Stock options
Class of Stock [Line Items]
Number of shares issued (in shares)		1,714,800
Options expiration after the date of grant											10 years
Stock-based compensation expense | $						$ 2,322,000			1,305,000		$ 6,305,000	$ 3,490,000
Stock options | Share-based Compensation Award, Tranche, First Anniversary of Grant Date
Class of Stock [Line Items]
Vesting percentage											100.00%
Stock options | Share Based Compensation Award Tranche One
Class of Stock [Line Items]
Vesting periods											1 year
Vesting percentage											25.00%
Stock options | Share Based Compensation Award Tranche Two
Class of Stock [Line Items]
Vesting periods											1 year
Number of equal quarterly installments | installment											12
Performance-Based Stock Options
Class of Stock [Line Items]
Number of shares granted (in shares)											0	0
Number of shares outstanding (in shares)						0					0		46,790
Performance-Based Stock Options | Keryx Biopharmaceuticals, Inc. | Merger Agreement
Class of Stock [Line Items]
Number of shares granted (in shares)				233,954
Restricted stock units
Class of Stock [Line Items]
Stock-based compensation expense | $						$ 3,990,000			1,217,000		$ 11,476,000	$ 3,318,000
Restricted stock units | Share Based Compensation Award Tranche One
Class of Stock [Line Items]
Vesting periods											1 year
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 periods											1 year
Vesting percentage											33.33%
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 periods											2 years
Vesting percentage											33.33%
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 periods											3 years
Vesting percentage											33.33%
Service-Based Restricted Stock Units
Class of Stock [Line Items]
Number of shares granted (in shares)		2,268,000
Stock-based compensation expense | $						3,900,000			1,200,000		$ 11,100,000	3,300,000
Service-Based Restricted Stock Units | Share Based Compensation Award Tranche One
Class of Stock [Line Items]
Vesting percentage											50.00%
Service-Based Restricted Stock Units | Share Based Compensation Award Tranche Two
Class of Stock [Line Items]
Vesting periods											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%
Performance Based Restricted Stock Unit
Class of Stock [Line Items]
Number of shares granted (in shares)		479,000
Stock-based compensation expense | $						$ 100,000			$ 0		$ 400,000	$ 0
Common Stock
Class of Stock [Line Items]
Common stock, shares outstanding (in shares)						143,328,652	143,129,409	130,251,440	118,863,735	118,787,301	143,328,652	118,863,735	121,674,568		117,122,262	116,887,518
Issuance of common stock, net of issuance costs (in shares)							12,650,000	7,973,967		1,384,520
Maximum | Stock options
Class of Stock [Line Items]
Vesting periods											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 periods											3 years
Minimum | Stock options
Class of Stock [Line Items]
Vesting periods											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 periods											1 year
At The Market Equity Offering Program
Class of Stock [Line Items]
Common stock sales agreement amount | $						$ 0					$ 0
At The Market Equity Offering Program | Common Stock
Class of Stock [Line Items]
Issuance of common stock, net of issuance costs (in shares)								7,973,967					2,684,392
Proceeds from the issuance of common stock, net of issuance costs | $								$ 56,700,000					$ 16,800,000
At The Market Equity Offering Program | Maximum
Class of Stock [Line Items]
Common stock sales agreement amount | $	$ 65,000,000.0		$ 75,000,000.0
2014 Plans
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)																	1,785,000
Incremental rate at which the shares reserved for issuance increase											3.00%
2014 Plans | Stock options | Employees
Class of Stock [Line Items]
Number of shares granted (in shares)											1,714,800
2014 Plans | Stock options | Director
Class of Stock [Line Items]
Number of shares granted (in shares)											220,900
2014 Plans | Restricted stock units | Employees
Class of Stock [Line Items]
Number of shares granted (in shares)											2,400,650
2014 Plans | Restricted stock units | Director
Class of Stock [Line Items]
Number of shares granted (in shares)											95,900
2014 Plans | Performance Based Restricted Stock Unit | Employees
Class of Stock [Line Items]
Number of shares granted (in shares)											515,500
2014 Employee Stock Purchase Plan
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)						5,480,334					5,480,334
Offering period											6 months
Purchase price at the end of offering period											85.00%
Number of shares issued (in shares)											235,658
Stock-based compensation expense | $						$ 280,000			$ 91,000		$ 591,000	$ 183,000