Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Equity - Shareholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, beginning balance (in shares)			3,577,103,522
Equity, beginning balance			$ 34,569	$ 40,308
Adoption of new accounting standards					$ 48
Net income attributable to Merck & Co., Inc.	$ 1,707	$ 1,946	2,443	3,496
Other comprehensive income (loss), net of taxes			62	132
Cash dividends declared on common stock			(2,592)	(2,583)
Treasury stock shares purchased			(2,162)	(2,153)
Share-based compensation plans and other			394	483
Acquisition of Vallée				25
Net income attributable to noncontrolling interests	$ 9	5	14	11
Distributions attributable to noncontrolling interests			$ (10)
Other changes in noncontrolling ownership interests				(7)
Shares, ending balance (in shares)	3,577,103,522		3,577,103,522
Equity, ending balance	$ 32,766	$ 39,712	$ 32,766	$ 39,712
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, beginning balance (in shares)			3,577,000,000	3,577,000,000
Equity, beginning balance			$ 1,788	$ 1,788
Shares, ending balance (in shares)	3,577,000,000	3,577,000,000	3,577,000,000	3,577,000,000
Equity, ending balance	$ 1,788	$ 1,788	$ 1,788	$ 1,788
Other Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity, beginning balance			39,902	39,939
Share-based compensation plans and other			(161)	(163)
Equity, ending balance	39,741	39,776	39,741	39,776
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity, beginning balance			41,350	44,133
Adoption of new accounting standards					322
Net income attributable to Merck & Co., Inc.			2,443	3,496
Cash dividends declared on common stock			(2,592)	(2,583)
Equity, ending balance	41,523	45,046	41,523	45,046
Accumulated Other Comprehensive Loss
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity, beginning balance			(4,910)	(5,226)
Adoption of new accounting standards					$ (274)
Other comprehensive income (loss), net of taxes			62	132
Equity, ending balance	$ (5,122)	$ (5,094)	$ (5,122)	$ (5,094)
Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, beginning balance (in shares)			880,000,000	828,000,000
Equity, beginning balance			$ (43,794)	$ (40,546)
Treasury stock shares purchased (in shares)			37,000,000	34,000,000
Treasury stock shares purchased			$ (2,162)	$ (2,153)
Share-based compensation plans and other (in shares)			(10,000,000)	(12,000,000)
Share-based compensation plans and other			$ 555	$ 646
Shares, ending balance (in shares)	907,000,000	850,000,000	907,000,000	850,000,000
Equity, ending balance	$ (45,401)	$ (42,053)	$ (45,401)	$ (42,053)
Non- Controlling Interests
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity, beginning balance			233	220
Acquisition of Vallée				25
Net income attributable to noncontrolling interests			14	11
Distributions attributable to noncontrolling interests			(10)
Other changes in noncontrolling ownership interests				(7)
Equity, ending balance	$ 237	$ 249	$ 237	$ 249