Stockholders' Equity - Additional Information (Details)						3 Months Ended		12 Months Ended
	Mar. 12, 2020 USD ($)	Feb. 28, 2020 shares	Nov. 12, 2019 USD ($)	Feb. 28, 2019 shares	Dec. 12, 2018 shares	Mar. 31, 2020 USD ($) Installment $ / shares shares	Mar. 31, 2019 USD ($) shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 shares	Feb. 28, 2014 shares
Class of Stock [Line Items]
Common stock, shares authorized						175,000,000		175,000,000
Common stock, par value | $ / shares						$ 0.00001		$ 0.00001
Common stock, shares issued						130,251,440		121,674,568
Common stock, shares outstanding						130,251,440		121,674,568
Preferred stock, shares authorized						25,000,000		25,000,000
Preferred stock, par value | $ / shares						$ 0.00001		$ 0.00001
Preferred stock, shares issued						0		0
Preferred stock, shares outstanding						0		0
Proceeds from the issuance of common stock, net of issuance costs | $						$ 56,485,000
Common stock, shares reserved for future issuance						24,958,932		21,403,890
Stock-based compensation expense | $						$ 4,916,000	$ 2,094,000
Inducement Award Program
Class of Stock [Line Items]
Granted, Shares						299,450
Outstanding, Shares						299,450
Inducement Award Program | Employees
Class of Stock [Line Items]
Granted, Shares						299,450
Stock Options
Class of Stock [Line Items]
Shares issued				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 | $						$ 1,617,000	1,112,000
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
Restricted Stock Units
Class of Stock [Line Items]
Granted, Shares		2,268,000
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 | $						$ 3,142,000	934,000
Restricted Stock Units | Share Based Compensation Award Tranche One
Class of Stock [Line Items]
Vesting percentage						50.00%
Restricted Stock Units | Share Based Compensation Award Tranche Two
Class of Stock [Line Items]
Vesting periods						6 months
Vesting percentage						25.00%
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]
Granted, Shares		479,000
Stock-based compensation expense | $						$ 100,000	$ 0
Performance-Based Stock Options
Class of Stock [Line Items]
Granted, Shares						0	0
Oustanding, 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]
Granted, Shares					233,954
Common Stock
Class of Stock [Line Items]
Common stock, shares outstanding						130,251,440	117,122,262	121,674,568	116,887,518
Issuance of common stock, net of issuance costs, shares						7,973,967
Minimum | Stock Options
Class of Stock [Line Items]
Vesting periods						12 months
Minimum | Restricted Stock Units
Class of Stock [Line Items]
Vesting periods						1 year
Minimum | Performance-Based Stock Options
Class of Stock [Line Items]
Potential share issuable percentage					0.00%
Maximum | Stock Options
Class of Stock [Line Items]
Vesting periods						48 months
Maximum | Restricted Stock Units
Class of Stock [Line Items]
Vesting periods						3 years
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%
At The Market Equity Offering Program
Class of Stock [Line Items]
Common stock sales agreement amount | $						$ 0
At The Market Equity Offering Program | Common Stock
Class of Stock [Line Items]
Issuance of common stock, net of issuance costs, 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										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]
Granted, Shares						1,714,800
2014 Plans | Stock Options | Director
Class of Stock [Line Items]
Granted, Shares						0
2014 Plans | Restricted Stock Units | Employees
Class of Stock [Line Items]
Granted, Shares						2,350,400
2014 Plans | Restricted Stock Units | Director
Class of Stock [Line Items]
Granted, Shares						0
2014 Plans | Performance Based Restricted Stock Unit | Employees
Class of Stock [Line Items]
Granted, Shares						479,000
2014 Employee Stock Purchase Plan
Class of Stock [Line Items]
Common stock, shares reserved for future issuance						5,600,968
Purchase price at the end of offering period						85.00%
Offering period						6 months
Shares issued						115,024
Stock-based compensation expense | $						$ 157,000	$ 48,000