Fair Value Measurements (Details) - USD ($)	3 Months Ended				9 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of liabilities	$ 8,045,000,000				$ 8,045,000,000		$ 8,362,000,000
Pre-tax charges recorded during period					9,000,000
Goodwill impairment charge	0		$ 0		350,000,000	$ 290,000,000
McKesson Europe Plan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pre-tax charges recorded during period					592,000,000
Goodwill impairment charge, after tax		$ 350,000,000
Distribution Solutions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill impairment charge					350,000,000
Technology Solutions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill impairment charge					0
Technology Solutions | Enterprise Information Solutions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill impairment charge			$ 290,000,000	$ 290,000,000
Goodwill impairment charge, after tax				$ 282,000,000
Fair value, measurements, recurring | Fair value, inputs, level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of liabilities	8,500,000,000				8,500,000,000		8,700,000,000
Fair value, measurements, recurring | Fair value, inputs, level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in money market funds	$ 1,066,000,000				$ 1,066,000,000		478,000,000
Fair value, measurements, nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value on a nonrecurring basis							$ 0
Intangible Asset and Store Assets Impairment | McKesson Europe Plan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pre-tax charges recorded during period		189,000,000
After-tax charges recorded		$ 157,000,000