Restructuring and Asset Impairment Charges - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		19 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Mar. 31, 2017
Restructuring Cost and Reserve [Line Items]
Restructuring and Asset Impairment Charges	$ 236	$ 0	$ 236	$ 0
Goodwill, Impairment Loss	350	290	350	290
McKesson Europe Plan
Restructuring Cost and Reserve [Line Items]
Restructuring and Asset Impairment Charges	586
Goodwill, Impairment Loss	350
McKesson Europe Plan | Intangible Asset and Store Assets Impairment
Restructuring Cost and Reserve [Line Items]
Restructuring and Asset Impairment Charges	189
After-tax charges recorded	157
McKesson Europe Plan | Severance
Restructuring Cost and Reserve [Line Items]
Restructuring and Asset Impairment Charges	47
After-tax charges recorded	40
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]
Restructuring and Asset Impairment Charges		(10)		(1)
Pre-tax restructuring charges incurred to-date					250
Cost Alignment Plan | Other Accrued Liabilities
Restructuring Cost and Reserve [Line Items]
Restructuring liabilities	59		59		59	$ 71
Cost Alignment Plan | Noncurrent Liabilities
Restructuring Cost and Reserve [Line Items]
Restructuring liabilities	31		31		$ 31	$ 35
Cost Alignment Plan | Severance
Restructuring Cost and Reserve [Line Items]
Payments for severance	$ 9	$ 26	$ 23	$ 71