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, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Earnings Per Share [Abstract]
Income from continuing operations	$ 1,072	$ 247	$ (676)	$ 483	$ (744)	$ 527	$ 552	$ (81)	$ 1,126	$ 254	$ 292
Net income attributable to noncontrolling interests									(220)	(221)	(230)
Income from continuing operations attributable to McKesson									906	33	62
Income (loss) from discontinued operations, net of tax	6	(5)	(1)	(6)	0	(1)	1	1	(6)	1	5
Net Income Attributable to McKesson Corporation	$ 1,021	$ 186	$ (730)	$ 423	$ (796)	$ 469	$ 499	$ (138)	$ 900	$ 34	$ 67
Weighted average common shares outstanding:
Basic (in shares)									181	196	208
Effect of dilutive securities (in shares)									1	1	1
Diluted (in shares)									182	197	209
Diluted
Continuing operations (in dollars per share)	$ 5.82	$ 1.06	$ (3.99)	$ 2.27	$ (4.17)	$ 2.41	$ 2.51	$ (0.69)	$ 4.99	$ 0.17	$ 0.30
Discontinued operations (in dollars per share)	0.03	(0.03)	0	(0.03)	0	(0.01)	0	0.01	(0.04)	0	0.02
Total (in dollars per share)	5.85	1.03	(3.99)	2.24	(4.17)	2.40	2.51	(0.68)	4.95	0.17	0.32
Basic
Continuing operations (in dollars per share)	5.86	1.06	(3.99)	2.28	(4.17)	2.42	2.52	(0.69)	5.01	0.17	0.30
Discontinued operations (in dollars per share)	0.03	(0.02)	0	(0.03)	0	(0.01)	0	0.01	(0.03)	0	0.02
Total (in dollars per share)	$ 5.89	$ 1.04	$ (3.99)	$ 2.25	$ (4.17)	$ 2.41	$ 2.52	$ (0.68)	$ 4.98	$ 0.17	$ 0.32