Stockholders’ Equity - Narrative (Details) - USD ($)	1 Months Ended				7 Months Ended	12 Months Ended
	Mar. 31, 2025	Feb. 28, 2025	Feb. 29, 2024	Apr. 30, 2023	Sep. 30, 2025	Jan. 02, 2026	Jan. 03, 2025	Dec. 29, 2023	Oct. 31, 2025	Aug. 31, 2024
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares earned (in shares)				1,000,000
Repurchases of common stock (in shares)						30,200,000
Repurchases of common stock						$ 1,159,700,000
Stock repurchase program, remaining authorized repurchase amount						590,200,000
August 2024 Repurchase Program
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock repurchase program, authorized amount										$ 500,000,000.0
February 2025 Repurchase Program
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock repurchase program, authorized amount		$ 500,000,000.0
October 2025 Repurchase Program
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock repurchase program, authorized amount									$ 750,000,000.0
Exelixis 401(k) Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Expenses relating to stock match						$ 12,800,000	$ 15,000,000.0	$ 13,900,000
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						$ 900,000
Unrecognized compensation expense weighted-average period for recognition						1 year 3 months 18 days
Intrinsic value of options exercised						$ 45,900,000	16,700,000	16,700,000
Cash received from option exercises and purchases under the ESPP						$ 36,100,000	$ 49,600,000	$ 20,800,000
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						$ 299,900,000
Unrecognized compensation expense weighted-average period for recognition						2 years 10 months 24 days
Number of awards granted (in shares)						11,751,000
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
Employee stock purchase plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares available for grant (in shares)						6,600,000
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)						400,000	700,000	900,000
Average price per share (in dollars per share)						$ 29.51	$ 18.23	$ 14.56
Proceeds from employee stock purchase plan						$ 13,100,000	$ 12,100,000	$ 12,700,000
TSR-Based Restricted Stock Units
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of awards granted (in shares)		1,000,000.0	1,300,000	800,000
Maximum potential to vest		175.00%	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%			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
Stock Price Target-Based Restricted Stock Units
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of awards granted (in shares)					7,200,000
Target price (in dollars per share)	$ 60
Performance stock units
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of awards granted (in shares)						0
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)						15,800,000
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