Earnings Per Common Share - Schedule of Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Income from continuing operations	$ (744)	$ 527	$ 552	$ (81)	$ (1,087)	$ 960	$ 56	$ 363	$ 254	$ 292	$ 5,277
Net income attributable to noncontrolling interests									(221)	(230)	(83)
Income from continuing operations attributable to McKesson									33	62	5,194
Income (Loss) from discontinued operations, net of tax	0	(1)	1	1	2	1	0	2	1	5	(124)
Net Income Attributable to McKesson Corporation	$ (796)	$ 469	$ 499	$ (138)	$ (1,146)	$ 903	$ 1	$ 309	$ 34	$ 67	$ 5,070
Weighted average common shares outstanding:
Basic (in shares)									196	208	221
Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Diluted (in shares)									197	209	223
Diluted
Continuing operations (in dollars per share)	$ (4.17)	$ 2.41	$ 2.51	$ (0.69)	$ (5.58)	$ 4.32	$ 0.01	$ 1.44	$ 0.17	$ 0.30	$ 23.28
Discontinued operations (in dollars per share)	0.00	(0.01)	0.00	0.01	0.00	0.01	0.00	0.01	0.00	0.02	(0.55)
Total (in dollars per share)	(4.17)	2.40	2.51	(0.68)	(5.58)	4.33	0.01	1.45	0.17	0.32	22.73
Basic
Continuing operations (in dollars per share)	(4.17)	2.42	2.52	(0.69)	(5.58)	4.34	0.01	1.46	0.17	0.30	23.50
Discontinued operations (in dollars per share)	0.00	(0.01)	0.00	0.01	0.00	0.01	0.00	0.00	0.00	0.02	(0.55)
Total (in dollars per share)	$ (4.17)	$ 2.41	$ 2.52	$ (0.68)	$ (5.58)	$ 4.35	$ 0.01	$ 1.46	$ 0.17	$ 0.32	$ 22.95
Options to purchase common stock
Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Effect of dilutive securities (in shares)									0	0	1
Restricted stock units
Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Effect of dilutive securities (in shares)									1	1	1