Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net Income Attributable to Merck & Co., Inc.	$ 2,394	$ 3,920	$ 4,442
Other Comprehensive Income (Loss) Net of Taxes:
Net unrealized loss on derivatives, net of reclassifications	(446)	(66)	(126)
Net unrealized loss on investments, net of reclassifications	(58)	(44)	(70)
Benefit plan net gain (loss) and prior service credit (cost), net of amortization	419	(799)	579
Cumulative translation adjustment	401	(169)	(208)
Other comprehensive income (loss), net of taxes	316	(1,078)	175
Comprehensive Income Attributable to Merck & Co., Inc.	$ 2,710	$ 2,842	$ 4,617