Earnings Per Share - Calculations of Earnings Per Share Under Two-Class Method (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Basic Earnings per Common Share
Net Income Attributable to Merck & Co., Inc.	$ 1,729	$ 1,692	$ 5,261	$ 4,760
Less: Income allocated to participating securities		3	4	12
Net income allocated to common shareholders	1,729	1,689	5,257	4,748
Average common shares outstanding	3,045	3,070	3,043	3,079
Basic Earnings per Common Share Attributable to Merck & Co., Inc. Common Shareholders	$ 0.57	$ 0.55	$ 1.73	$ 1.54
Earnings per Common Share Assuming Dilution
Net Income Attributable to Merck & Co., Inc.	1,729	1,692	5,261	4,760
Less: Income allocated to participating securities		3	4	12
Net income allocated to common shareholders	$ 1,729	$ 1,689	$ 5,257	$ 4,748
Average common shares outstanding	3,045	3,070	3,043	3,079
Common shares issuable	34	21	34	23
Average common shares outstanding assuming dilution	3,079	3,091	3,077	3,102
Earnings per Common Share Assuming Dilution Attributable to Merck & Co., Inc. Common Shareholders	$ 0.56	$ 0.55	$ 1.71	$ 1.53