Supplemental Income Statement Information - Schedule of Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 28, 2025	Jun. 29, 2024	Jun. 28, 2025	Jun. 29, 2024
Supplemental Income Statement Elements [Abstract]
Net income attributable to Thermo Fisher Scientific Inc.	$ 1,617	$ 1,548	$ 3,124	$ 2,875
Basic weighted average shares (in shares)	378	382	378	382
Plus effect of: stock options and restricted stock units (in shares)	0	1	1	1
Diluted weighted average shares (in shares)	378	383	378	383
Basic earnings per share (in dollars per share)	$ 4.28	$ 4.05	$ 8.27	$ 7.53
Diluted earnings per share (in dollars per share)	$ 4.28	$ 4.04	$ 8.26	$ 7.50
Antidilutive stock options excluded from diluted weighted average shares (in shares)	4	2	3	2