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, 2020	Mar. 31, 2019	Mar. 31, 2018
Schedule of Equity Method Investments [Line Items]
Estimated net gain on split-off of the Change Healthcare JV		$ 0	$ 56	$ 43
Change Healthcare
Schedule of Equity Method Investments [Line Items]
Fair value of McKesson common stock accepted (15,426,537 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)
Transaction costs incurred	(23)
Estimated net gain on split-off of the Change Healthcare JV	$ 414
Separation of Change Healthcare JV
Schedule of Equity Method Investments [Line Items]
Shares repurchased (in shares)	15.4
Price paid per share (in dollars per share)	$ 131.97