Business Combination - Additional Information (Details)			3 Months Ended			6 Months Ended	9 Months Ended		12 Months Ended
	Dec. 12, 2018 USD ($)	Dec. 11, 2018 USD ($) shares	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Business Acquisition [Line Items]
Impairment of intangible asset			$ 0	$ 115,500,000	$ 0		$ 115,527,000	$ 0
Increase in excess inventory purchase commitments							$ 10,304,000	$ 0
Developed product rights for Auryxia
Business Acquisition [Line Items]
Intangible assets, estimated useful life			7 years			9 years	7 years		9 years
Keryx Biopharmaceuticals, Inc.
Business Acquisition [Line Items]
Risk-adjusted discount rate used to calculate present value of future expected cash inflows of intangible assets	20.00%
Preliminary fair value of the off-market element	$ 29,510,000		$ 41,000,000.0	41,500,000		$ 41,500,000	$ 41,000,000.0		$ 30,200,000
Increase in excess inventory purchase commitments				$ 11,000,000.0
Goodwill deductible for tax purposes	0
Deferred tax liability	$ 35,096,000
Keryx Biopharmaceuticals, Inc. | Developed product rights for Auryxia
Business Acquisition [Line Items]
Intangible assets, estimated useful life	9 years		7 years
Keryx Biopharmaceuticals, Inc. | Notes Conversion Agreement
Business Acquisition [Line Items]
Convertible notes amount		$ 164,700,000
Keryx Biopharmaceuticals, Inc. | Notes Conversion Agreement | Common Stock
Business Acquisition [Line Items]
Convertible notes converted into shares (in shares) | shares		35,582,335
Keryx Biopharmaceuticals, Inc. | Merger Agreement
Business Acquisition [Line Items]
Additional shares expected to be issued (in shares) | shares		4,000,000
Aggregate shares available for conversion (in shares) | shares		39,600,000
Outstanding common stock and awards converted into right to receive shares and awards, ratio	0.37433	0.37433
Fair value of additional shares issued		$ 13,400,000