Stockholders' Equity - Additional Information (Details)								3 Months Ended			6 Months Ended		12 Months Ended
	May 19, 2020 USD ($) $ / shares shares	May 19, 2020 USD ($) $ / shares	May 14, 2020 USD ($) $ / shares shares	Mar. 12, 2020 USD ($)	Feb. 28, 2020 shares	Nov. 12, 2019 USD ($)	Dec. 12, 2018 shares	Jun. 30, 2020 USD ($) $ / shares shares	Mar. 31, 2020 USD ($) shares	Jun. 30, 2019 USD ($) shares	Jun. 30, 2020 USD ($) installment $ / shares shares	Jun. 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)								143,129,409			143,129,409		121,674,568
Common stock, shares outstanding (in shares)								143,129,409			143,129,409		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)								24,926,906			24,926,906		21,403,890
Stock-based compensation expense | $								$ 6,864,000		$ 2,284,000	$ 11,780,000	4,378,000
Follow-On Public Offering
Class of Stock [Line Items]
Number of shares sold in offering (in shares)			11,000,000
Offering price per share (in dollars per share) | $ / shares			$ 12.00
Aggregate net proceeds from offering | $		$ 142,400,000	$ 123,800,000
Over-Allotment Option
Class of Stock [Line Items]
Number of shares sold in offering (in shares)	1,650,000
Offering price per share (in dollars per share) | $ / shares	$ 12.00	$ 12.00
Aggregate net proceeds from offering | $	$ 18,600,000
Inducement Award Program
Class of Stock [Line Items]
Number of shares granted (in shares)											786,750
Number of shares outstanding (in shares)								786,750			786,750
Inducement Award Program | Employees
Class of Stock [Line Items]
Number of shares granted (in shares)											786,750
Stock options
Class of Stock [Line Items]
Number of shares issued (in shares)					1,714,800
Stock-based compensation description											Options vest either 100% on the first anniversary of the grant date or in installments of (i) 25% at the one year anniversary and (ii) 12 equal quarterly installments beginning after the one year anniversary of the grant date, subject to the individual’s continuous service with the Company
Options expiration after the date of grant											10 years
Stock-based compensation expense | $								$ 2,366,000		1,073,000	$ 3,983,000	$ 2,185,000
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]
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
Vesting percentage, Following achievement of performance condition							50.00%
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 description											Generally, RSUs granted by the Company vest in one of the following ways: (i) 100% of each RSU grant vests on either the first or the third anniversary of the grant date, (ii) one third of each RSU grant vests on the first, second and third anniversaries of the grant date, subject, in each case, to the individual’s continued service through the applicable vesting date, or (iii) 50% of each RSU grant vests on the first anniversary and 25% of each RSU grant vests in 6 months increment after the one year anniversary of the grant date.
Stock-based compensation expense | $								$ 4,344,000		1,168,000	$ 7,486,000	$ 2,102,000
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 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 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 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 | $								4,200,000		1,200,000	$ 7,200,000	2,100,000
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 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%
Performance Based Restricted Stock Unit
Class of Stock [Line Items]
Number of shares granted (in shares)					479,000
Stock-based compensation expense | $								$ 200,000		$ 0	$ 300,000	$ 0
Common Stock
Class of Stock [Line Items]
Common stock, shares outstanding (in shares)								143,129,409	130,251,440	118,787,301	143,129,409	118,787,301	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 | Performance-Based Stock Options
Class of Stock [Line Items]
Potential share issuable percentage							100.00%
Vesting percentage, upon achievement of performance condition							50.00%
Maximum | Restricted stock units
Class of Stock [Line Items]
Vesting periods											3 years
Minimum | Stock options
Class of Stock [Line Items]
Vesting periods											12 months
Minimum | Performance-Based Stock Options
Class of Stock [Line Items]
Potential share issuable percentage							0.00%
Minimum | Restricted stock units
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)											2,367,500
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)											479,000
2014 Employee Stock Purchase Plan
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)								5,600,968			5,600,968
Offering period											6 months
Purchase price at the end of offering period											85.00%
Number of shares issued (in shares)											115,024
Stock-based compensation expense | $								$ 154,000		$ 43,000	$ 311,000	$ 91,000