Investment in Change Healthcare Joint Venture - Net Gain on Split-off of Change Healthcare JV (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		12 Months Ended
	Mar. 09, 2020	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Schedule of Equity Method Investments [Line Items]
Net gain on split-off of the Change Healthcare JV		$ 0	$ 0	$ 56
Separation of Change Healthcare JV
Schedule of Equity Method Investments [Line Items]
Shares of common stock exchanged (shares)	15.4
Change Healthcare
Schedule of Equity Method Investments [Line Items]
Fair value of McKesson common stock accepted (15.4 million shares at $131.97 per share on March 9, 2020)	$ 2,036
Investment in the Change Healthcare JV at exchange date	(2,096)
Reversal of deferred tax liability	521
Release of accumulated other comprehensive attributable to the joint venture	(24)
Less: Transaction costs incurred	(23)
Net gain on split-off of the Change Healthcare JV	$ 414
Shares repurchased (in shares)	15.4
Cost of shares repurchased (in usd per share)	$ 131.97