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STOCK-BASED COMPENSATION AND EMPLOYEE RETIREMENT PLANS - Narrative (Details) $ in Thousands |  | 12 Months Ended
--- | --- | ---
Jun. 06, 2019 USD ($) shares | Dec. 31, 2025 USD ($) installment age shares | Dec. 31, 2024 USD ($) shares | Dec. 31, 2023 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Stock-based compensation expense |  | $ 11,283 | $ 7,775 | $ 9,317
Granted (in shares) | shares |  | 5,464,077 |  |
Number of options outstanding (in shares) | shares |  | 18,152,258 | 16,684,325 |
Options exercised, aggregate intrinsic value |  | $ 1,800 | $ 400 | 0
Fair value of options vested |  | 4,300 | 3,700 | 6,400
Unrecognized compensation costs, options |  | $ 11,200 |  |
Unrecognized compensation costs, options, period for recognition (in years) |  | 2 years 7 months 20 days |  |
Employees eligibility age to participate in retirement plan | age |  | 21 |  |
Employers contributions to defined contribution plan |  | $ 1,900 | 1,700 | 1,700
2014 Employee Stock Purchase Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Stock-based compensation expense |  | $ 168 | $ 84 | 73
Purchase price at the end of offering period | 85.00% |  |  |
Offering period (in months) | 6 months |  |  |
Maximum employee subscription rate (as percent) | 15.00% |  |  |
Maximum employee subscription amount | $ 25 |  |  |
Maximum number of shares per employee in offering period | shares | 1,500 |  |  |
Common stock, shares reserved for future issuance (in shares) | shares |  | 4,260,647 | 4,448,069 |
Number of shares issued (in shares) | shares |  | 187,422 |  |
Share-based Payment Arrangement, Option |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Stock-based compensation expense |  | $ 5,356 | $ 4,036 | 5,310
Expiration period (in years) |  | 10 years |  |
Share-based Payment Arrangement, Option | Share-based Compensation Award, Tranche, First Anniversary of Grant Date |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting percentage |  | 100.00% |  |
Share-based Payment Arrangement, Option | Share Based Compensation Award Tranche One |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting period |  | 1 year |  |
Vesting percentage |  | 25.00% |  |
Share-based Payment Arrangement, Option | Share Based Compensation Award Tranche Two |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting period |  | 1 year |  |
Number of equal quarterly installments | installment |  | 12 |  |
Share-based Payment Arrangement, Option | Minimum |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting period |  | 12 months |  |
Share-based Payment Arrangement, Option | Maximum |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting period |  | 48 months |  |
Stock appreciation rights |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Expiration period (in years) |  | 10 years |  |
Stock appreciation rights | Share Based Compensation Award Tranche One |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting period |  | 1 year |  |
Vesting percentage |  | 25.00% |  |
Stock appreciation rights | Share Based Compensation Award Tranche Two |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting period |  | 1 year |  |
Number of equal quarterly installments | installment |  | 12 |  |
Stock appreciation rights | Minimum |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting period |  | 12 months |  |
Stock appreciation rights | Maximum |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting period |  | 48 months |  |
Inducement Award Program |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Granted (in shares) | shares |  | 1,454,477 |  |
Number of options outstanding (in shares) | shares |  | 1,152,477 |  |
Inducement Award Program | Akebia Therapeutics, Inc. 2023 Stock Incentive Plan (the 2023 Plan) (replaces 2014 Plan) |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Number of options outstanding (in shares) | shares |  | 3,034,085 | 2,534,775 |
Unvested restricted stock units |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Stock-based compensation expense |  | $ 5,214 | $ 3,655 | 3,637
Unvested restricted stock units | Share-based Compensation Award, Tranche, First Anniversary of Grant Date |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting percentage |  | 100.00% |  |
Unvested restricted stock units | Share-Based Payment Arrangement, Tranche One, First Anniversary After Grant Date, Subjected To Individual Continued Service |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting percentage |  | 33.33% |  |
Unvested restricted stock units | Share-Based Payment Arrangement, Tranche Two, Second Anniversary After Grant Date, Subjected To Individual Continued Service |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting percentage |  | 33.33% |  |
Unvested restricted stock units | Share-Based Payment Arrangement, Tranche Three, Third Anniversary After Grant Date, Subjected To Individual Continued Service |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting percentage |  | 33.33% |  |
Unvested restricted stock units | Share-Based Payment Arrangement, Tranche One, First Anniversary After Grant Date |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting percentage |  | 33.33% |  |
Unvested restricted stock units | Share-Based Payment Arrangement, Tranche Two, Following First Anniversary After Grant Date |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting period |  | 2 years |  |
Vesting percentage |  | 66.67% |  |
Number of equal quarterly installments | installment |  | 8 |  |
Award vesting grace period (in years) |  | 1 year |  |
Unvested restricted stock units | Share Based Compensation Award Tranche One |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting percentage |  | 50.00% |  |
Unvested restricted stock units | Share Based Compensation Award Tranche Two |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting period |  | 6 months |  |
Vesting percentage |  | 25.00% |  |
Award vesting grace period (in years) |  | 1 year |  |
Restricted Stock Units (RSUs) And Performance Based Restricted Stock Units (PSUs) |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Unrecognized compensation costs, options, period for recognition (in years) |  | 1 year 8 months 23 days |  |
Vested in period |  | $ 3,400 | $ 3,200 | $ 8,400
Unrecognized compensation costs, stock |  | $ 7,700 |  |
Employees | Share-based Payment Arrangement, Option | Akebia Therapeutics, Inc. 2023 Stock Incentive Plan (the 2023 Plan) (replaces 2014 Plan) |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Granted (in shares) | shares |  | 3,634,300 |  |
Director | Share-based Payment Arrangement, Option | Akebia Therapeutics, Inc. 2023 Stock Incentive Plan (the 2023 Plan) (replaces 2014 Plan) |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Granted (in shares) | shares |  | 375,200 |  |

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