Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ (1,934)	$ 7,005	$ 5,486	$ 7,035
Other comprehensive income (loss), pretax	(26)	44	128	(75)
Tax	0	0	(10)	(3)
Other comprehensive income (loss), net of taxes	(26)	44	118	(78)
Net transfer of benefit plans to (from) Merck affiliates	(3)		11
Ending balance	(1,618)	7,335	(1,618)	7,335
Accumulated Other Comprehensive (Loss) Income
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(464)	(1,036)	(622)	(914)
Ending balance	(493)	(992)	(493)	(992)
Employee Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(26)	(343)	(32)	(354)
Other comprehensive income (loss), pretax	(3)	4	(1)	18
Tax	0	0	(10)	(3)
Other comprehensive income (loss), net of taxes	(3)	4	(11)	15
Net transfer of benefit plans to (from) Merck affiliates	(3)		11
Ending balance	(32)	(339)	(32)	(339)
Accumulated Foreign Currency Adjustment Attributable to Parent
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(438)	(693)	(590)	(560)
Other comprehensive income (loss), pretax	(23)	40	129	(93)
Other comprehensive income (loss), net of taxes	(23)	40	129	(93)
Ending balance	$ (461)	$ (653)	$ (461)	$ (653)