Strategic Merger with Akebia Therapeutics, Inc. - Narrative (Details) $ / shares in Units, shares in Millions	Jun. 28, 2018 USD ($) $ / shares shares	May 09, 2018 shares	Jun. 30, 2018 $ / shares	Dec. 31, 2017 $ / shares
Business Acquisition [Line Items]
Common stock, par value (in dollars per share) | $ / shares			$ 0.001	$ 0.001
Additional shares issued (in shares)		2.2
Keryx Biopharmaceuticals
Business Acquisition [Line Items]
Contingent termination fee | $	$ 22,000,000
Akebia Therapeutics | Keryx Biopharmaceuticals
Business Acquisition [Line Items]
Conversion rate of acquiree stock	0.37433
Common stock, par value (in dollars per share) | $ / shares	$ 0.00001
Voting rights percentage	50.00%
Contingent termination fee | $	$ 22,000,000.0
Convertible Debt | Convertible Senior Notes due 2021
Business Acquisition [Line Items]
Additional shares issued (in shares)	4.0	4.0
Shares issued in conversion of debt (in shares)		35.6