Stockholders’ Equity - Estimated Grant-Date Fair Value (Details) - $ / shares	12 Months Ended
	Jan. 02, 2026	Jan. 03, 2025	Dec. 29, 2023
TSR-Based Restricted Stock Units
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Weighted average grant date fair value, awarded (in dollars per share)	$ 47.58	$ 20.19	$ 26.05
Fair value of Exelixis common stock on grant date (in dollars per share)	$ 37.53	$ 21.71	$ 19.48
Expected volatility	33.00%	37.00%	40.00%
Risk-free interest rate	4.00%	4.40%	3.80%
Dividend yield	0.00%	0.00%	0.00%
Stock Price Target-Based Restricted Stock Units
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Weighted average grant date fair value, awarded (in dollars per share)	$ 25.26
Fair value of Exelixis common stock on grant date (in dollars per share)	$ 37.09
Expected volatility	38.00%
Risk-free interest rate	3.90%
Dividend yield	0.00%