STOCK-BASED COMPENSATION AND BENEFIT PLAN - Equity Plans (Details) - shares	Mar. 31, 2024	Dec. 31, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of options outstanding (in shares)	16,877,776	13,312,835
Inducement Award Program
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of options outstanding (in shares)	970,950
Keryx Equity Plans
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Awards outstanding (in shares)	224,929	232,203
Additional awards authorized for grant (in shares)	0	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)	13,531,330	15,311,501
Additional awards authorized for grant (in shares)	0	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 options outstanding (in shares)	1,525,375	1,616,019
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)	9,198,750	1,712,400
Additional awards authorized for grant (in shares)	10,945,556	17,382,722
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 options outstanding (in shares)	1,764,950	794,000