Earnings Per Common Share (Basic and Diluted Earnings per Common Share) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended				9 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2014		Dec. 31, 2013
Schedule of Earnings Per Share [Line Items]
Income from continuing operations	$ 513		$ 164		$ 1,429		$ 1,015
Net income attributable to noncontrolling interests	(39)		0		(55)		0
Income from continuing operations attributable to McKesson	474		164		1,374		1,015
Loss from discontinued operations, net of tax	(2)		(99)		(30)		(122)
Net income attributable to McKesson	$ 472		$ 65		$ 1,344		$ 893
Weighted average common shares outstanding:
Basic	232		230		232		229
Effect of dilutive securities:
Options to purchase common stock	2		2		1		1
Restricted stock units	2		2		2		3
Diluted	236		234		235		233
Earnings (Loss) Per Common Share Attributable To McKesson (Diluted)
Continuing operations	$ 2.01	[1]	$ 0.70	[1]	$ 5.84	[1]	$ 4.36	[1]
Discontinued operations	$ (0.01)	[1]	$ (0.42)	[1]	$ (0.12)	[1]	$ (0.53)	[1]
Total	$ 2.00	[1]	$ 0.28	[1]	$ 5.72	[1]	$ 3.83	[1]
Earnings (Loss) Per Common Share Attributable To McKesson (Basic)
Continuing operations	$ 2.04	[1]	$ 0.71	[1]	$ 5.93	[1]	$ 4.43	[1]
Discontinued operations	$ (0.01)	[1]	$ (0.43)	[1]	$ (0.13)	[1]	$ (0.53)	[1]
Total	$ 2.03	[1]	$ 0.28	[1]	$ 5.80	[1]	$ 3.90	[1]

[1] Certain computations may reflect rounding adjustments.