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, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share [Abstract]
Income (loss) from continuing operations	$ 713	$ (6,174)	$ 627	$ 495	$ 1,072	$ 247	$ (676)	$ 483	$ (4,339)	$ 1,126	$ 254
Net income attributable to noncontrolling interests									(199)	(220)	(221)
Income (loss) from continuing operations attributable to McKesson									(4,538)	906	33
Income (loss) from discontinued operations, net of tax	0	0	0	(1)	6	(5)	(1)	(6)	(1)	(6)	1
Net income (loss) attributable to McKesson Corporation	$ 666	$ (6,226)	$ 577	$ 444	$ 1,021	$ 186	$ (730)	$ 423	$ (4,539)	$ 900	$ 34
Weighted-average common shares outstanding:
Basic (in shares)									160.6	180.6	196.3
Effect of dilutive securities (in shares)									0.0	1.0	1.0
Diluted (in shares)									160.6	181.6	197.3
Diluted
Continuing operations (in dollars per share)	$ 4.15	$ (39.03)	$ 3.54	$ 2.72	$ 5.82	$ 1.06	$ (3.99)	$ 2.27	$ (28.26)	$ 4.99	$ 0.17
Discontinued operations (in dollars per share)	0	0	0	0	0.03	(0.03)	0	(0.03)	0	(0.04)	0
Total (in dollars per share)	4.15	(39.03)	3.54	2.72	5.85	1.03	(3.99)	2.24	(28.26)	4.95	0.17
Basic
Continuing operations (in dollars per share)	4.19	(39.03)	3.56	2.74	5.86	1.06	(3.99)	2.28	(28.26)	5.01	0.17
Discontinued operations (in dollars per share)	0	0	0	0	0.03	(0.02)	0	(0.03)	0	(0.03)	0
Total (in dollars per share)	$ 4.19	$ (39.03)	$ 3.56	$ 2.74	$ 5.89	$ 1.04	$ (3.99)	$ 2.25	$ (28.26)	$ 4.98	$ 0.17