CAPITAL STOCK, STOCK-BASED COMPENSATION AND BENEFIT PLAN - Equity Plans (Details) - shares	Sep. 30, 2023	Dec. 31, 2022
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares outstanding	14,614,113	11,775,411
Inducement Award Program
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares outstanding	701,000
Keryx Equity Plans
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Awards outstanding (in shares)	284,556
Additional awards authorized for grant (in shares)	0
Akebia Therapeutics, Inc. Amended and Restated 2008 Equity Incentive Plan (the 2008 Plan)
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Awards outstanding (in shares)	419
Additional awards authorized for grant (in shares)	0
Akebia Therapeutics, Inc. 2014 Incentive Plan, as amended (the 2014 Plan) (replaces 2008 Plan)
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Awards outstanding (in shares)	17,033,234
Additional awards authorized for grant (in shares)	0
Akebia Therapeutics, Inc. 2014 Incentive Plan, as amended (the 2014 Plan) (replaces 2008 Plan) | Inducement Award Program
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares outstanding	2,035,832
Akebia Therapeutics, Inc. 2023 Stock Incentive Plan (the 2023 Plan) (replaces 2014 Plan)
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Awards outstanding (in shares)	1,558,500
Additional awards authorized for grant (in shares)	16,169,791
Akebia Therapeutics, Inc. 2023 Stock Incentive Plan (the 2023 Plan) (replaces 2014 Plan) | Inducement Award Program
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares outstanding	653,000