Supplemental Income Statement Information - Schedule of Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 29, 2025	Mar. 30, 2024
Supplemental Income Statement Elements [Abstract]
Net income attributable to Thermo Fisher Scientific Inc.	$ 1,507	$ 1,328
Basic weighted average shares (in shares)	378	382
Plus effect of: stock options and restricted stock units (in shares)	1	2
Diluted weighted average shares (in shares)	379	384
Basic earnings per share (in dollars per share)	$ 3.99	$ 3.47
Diluted earnings per share (in dollars per share)	$ 3.98	$ 3.46
Antidilutive stock options excluded from diluted weighted average shares (in shares)	3	2