CONDENSED CONSOLIDATED STATEMENT OF INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Sales	$ 12,551	$ 12,397	$ 35,479	$ 34,972
Costs, Expenses and Other
Cost of sales	3,481	3,990	9,952	10,443
Selling, general and administrative	2,450	2,589	7,383	7,726
Research and development	3,390	3,204	7,721	7,324
Restructuring costs	114	232	269	444
Other (income) expense, net	(312)	35	(630)	362
Total Costs, Expenses and Other	9,123	10,050	24,695	26,299
Income Before Taxes	3,428	2,347	10,784	8,673
Taxes on Income	483	440	1,611	1,259
Net Income	2,945	1,907	9,173	7,414
Less: Net Income (Loss) Attributable to Noncontrolling Interests	4	6	12	(73)
Net Income Attributable to Merck & Co., Inc.	$ 2,941	$ 1,901	$ 9,161	$ 7,487
Basic Earnings per Common Share Attributable to Merck & Co., Inc. Common Shareholders (in dollars per share)	$ 1.16	$ 0.74	$ 3.62	$ 2.91
Earnings per Common Share Assuming Dilution Attributable to Merck & Co., Inc. Common Shareholders (in dollars per share)	$ 1.16	$ 0.74	$ 3.61	$ 2.89