Restructuring and Asset Impairment Charges - Narrative (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		24 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Mar. 31, 2017
Restructuring Cost and Reserve [Line Items]
Net restructuring charges recognized				$ 9
Goodwill impairment charge	$ 0		$ 0	350	$ 290
Restructuring liabilities	80			80		$ 80	$ 106
Other Accrued Liabilities
Restructuring Cost and Reserve [Line Items]
Restructuring liabilities	51			51		51
Noncurrent Liabilities
Restructuring Cost and Reserve [Line Items]
Restructuring liabilities	29			29		29
McKesson Europe Plan
Restructuring Cost and Reserve [Line Items]
Net restructuring charges recognized				592
McKesson Europe Plan | Intangible Asset and Store Assets Impairment
Restructuring Cost and Reserve [Line Items]
Net restructuring charges recognized		$ 189
After-tax charges recorded		$ 157
McKesson Europe Plan | Severance and Lease exit costs
Restructuring Cost and Reserve [Line Items]
Net restructuring charges recognized	6			53
After-tax charges recorded	5			45
McKesson Europe Plan | Minimum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	650			650		650
McKesson Europe Plan | Maximum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	$ 750			$ 750		750
Cost Alignment Plan
Restructuring Cost and Reserve [Line Items]
Pre-tax restructuring charges incurred to-date						$ 252