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Stockholders’ Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands | 1 Months Ended | 3 Months Ended | 12 Months Ended |
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Apr. 30, 2023 | Mar. 31, 2022 | Mar. 31, 2021 | Sep. 29, 2023 | Mar. 31, 2023 | Sep. 30, 2022 | Oct. 01, 2021 | Dec. 29, 2023 | Dec. 30, 2022 | Dec. 31, 2021 | Jan. 01, 2021 | Jan. 31, 2024
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Maximum potential to vest |  |  |  | 50.00% |  | 25.00% | 25.00% |  |  |  |  |
Repurchases of common stock |  |  |  |  |  |  |  | $ 554,870 |  |  |  |
Exelixis 401(k) Plan |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Expenses relating to stock match |  |  |  |  |  |  |  | $ 13,900 | $ 11,700 | $ 9,500 |  |
March 2023 Repurchase Program |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchase program, authorized amount |  |  |  |  | $ 550,000 |  |  |  |  |  |  |
Repurchases of common stock (in shares) |  |  |  |  |  |  |  | 26,200,000 |  |  |  |
Repurchases of common stock |  |  |  |  |  |  |  | $ 550,000 |  |  |  |
January 2024 Repurchase Program | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchase program, authorized amount |  |  |  |  |  |  |  |  |  |  |  | $ 450,000
Minimum |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of months following change in control in which an employee was terminated |  |  |  |  |  |  |  | 3 months |  |  |  |
Maximum |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of months following change in control in which an employee was terminated |  |  |  |  |  |  |  | 15 months |  |  |  |
Stock options |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Expiration period |  |  |  |  |  |  |  | 7 years |  |  |  |
Total unrecognized compensation expense |  |  |  |  |  |  |  | $ 9,800 |  |  |  |
Unrecognized compensation expense weighted-average period for recognition |  |  |  |  |  |  |  | 1 year 10 months 24 days |  |  |  |
Intrinsic value of options exercised |  |  |  |  |  |  |  | $ 16,700 | 36,500 | 76,000 |  |
Cash received from option exercises and purchases under the ESPP |  |  |  |  |  |  |  | $ 20,800 | $ 13,900 | $ 14,800 |  |
Stock options | Share-based Payment Arrangement, Employee |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Award vesting period |  |  |  |  |  |  |  | 4 years |  |  |  |
Stock options | Share-based Payment Arrangement, Nonemployee |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Award vesting period |  |  |  |  |  |  |  | 1 year |  |  |  |
Restricted stock units |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total unrecognized compensation expense |  |  |  |  |  |  |  | $ 172,700 |  |  |  |
Unrecognized compensation expense weighted-average period for recognition |  |  |  |  |  |  |  | 2 years 6 months |  |  |  |
Number of awards granted (in shares) |  |  |  |  |  |  |  | 5,028,000 |  |  |  |
Number of awards (in shares) |  |  |  |  |  |  |  | 11,874,000 | 11,294,000 |  |  |
Restricted stock units | Share-based Payment Arrangement, Employee |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Award vesting period |  |  |  |  |  |  |  | 4 years |  |  |  |
Restricted stock units | Share-based Payment Arrangement, Nonemployee |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Award vesting period |  |  |  |  |  |  |  | 1 year |  |  |  |
TSR-Based Restricted Stock Units |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of awards granted (in shares) | 800,000 | 1,000,000 |  |  |  |  |  |  |  |  |  |
Maximum potential to vest | 175.00% | 175.00% |  |  |  |  |  |  |  |  |  |
TSR-Based Restricted Stock Units | Share-based Payment Arrangement, Tranche One |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Vesting award percentage | 50.00% | 50.00% |  |  |  |  |  |  |  |  |  |
TSR-Based Restricted Stock Units | Share-based Payment Arrangement, Tranche Two |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Award vesting period | 1 year | 1 year |  |  |  |  |  |  |  |  |  |
ESPP |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of shares available for grant (in shares) |  |  |  |  |  |  |  | 1,700,000 |  |  |  |
Discount rate from market value on purchase date (as a percent) |  |  |  |  |  |  |  | 85.00% |  |  |  |
Discount rate from market value on offering date (as a percent) |  |  |  |  |  |  |  | 85.00% |  |  |  |
Purchase period |  |  |  |  |  |  |  | 6 months |  |  |  |
Common stock issued (in shares) |  |  |  |  |  |  |  | 900,000 | 600,000 | 500,000 |  |
Average price per share (in dollars per share) |  |  |  |  |  |  |  | $ 14.56 | $ 16.63 | $ 17.76 |  |
Proceeds from employee stock purchase plan |  |  |  |  |  |  |  | $ 12,700 | $ 10,100 | $ 9,500 |  |
Performance stock units |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Unrecognized compensation expense weighted-average period for recognition |  |  |  |  |  |  |  | 1 year 3 months 18 days |  |  |  |
Number of awards granted (in shares) |  |  | 1,000,000 |  |  |  |  | 0 |  |  |  |
Maximum potential to vest |  |  | 200.00% |  | 100.00% |  |  | 125.00% |  |  | 200.00% |
Additional shares to be earned (in shares) |  |  | 2,100,000 |  |  |  |  |  |  |  | 4,700,000 |
Number of shares earned |  |  |  |  |  |  |  | 1,000,000 |  |  |  |
Number of awards (in shares) |  |  |  |  |  |  |  | 3,891,000 | 4,964,000 |  |  |
Performance stock units | Achieved |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of awards (in shares) |  |  |  |  |  |  |  | 1,500,000 |  |  |  |
Performance stock units | Probable |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total unrecognized compensation expense |  |  |  |  |  |  |  | $ 6,000 |  |  |  |
Performance stock units | Not Probable |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total unrecognized compensation expense |  |  |  |  |  |  |  | $ 41,400 |  |  |  |
Performance stock units | Share-based Payment Arrangement, Tranche One |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Vesting award percentage |  |  | 50.00% |  |  |  |  |  |  |  |  |
Performance stock units | Share-based Payment Arrangement, Tranche Two |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Award vesting period |  |  | 1 year |  |  |  |  |  |  |  |  |
Performance Shares, Clinical Trial Results |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of awards granted (in shares) |  |  |  |  |  |  |  |  |  |  | 2,300,000 |
Amended and Restated 2017 Equity Incentive Plan |  |  |  |  |  |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of shares available for grant (in shares) |  |  |  |  |  |  |  | 27,900,000 |  |  |  |
Reduction in share reserve after stock option or stock appreciation award (in shares) |  |  |  |  |  |  |  | 1 |  |  |  |
Reduction in share reserve after all other awards (in shares) |  |  |  |  |  |  |  | 2 |  |  |  |

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