Earnings per Share - Schedule of Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 28, 2024	Sep. 30, 2023	Sep. 28, 2024	Sep. 30, 2023
Earnings Per Share [Abstract]
Net income attributable to Thermo Fisher Scientific Inc.	$ 1,630	$ 1,715	$ 4,505	$ 4,365
Basic weighted average shares (in shares)	382	386	382	386
Plus effect of: stock options and restricted stock units (in shares)	1	2	1	2
Diluted weighted average shares (in shares)	384	388	383	388
Basic earnings per share (in dollars per share)	$ 4.26	$ 4.44	$ 11.79	$ 11.31
Diluted earnings per share (in dollars per share)	$ 4.25	$ 4.42	$ 11.75	$ 11.25
Antidilutive stock options excluded from diluted weighted average shares (in shares)	2	2	2	2