Restructuring and Asset Impairment Charges - Narrative (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended		16 Months Ended
	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Mar. 31, 2018
Restructuring Cost and Reserve [Line Items]
Pre-tax restructuring and asset impairment charges	$ 110		$ 6		$ 288	$ 242
After-tax restructuring and asset impairment charges	92		5		244	202
Pre-tax impairment charge		$ 20
After-tax impairment charges		$ 16
Impairment of long-lived and intangible assets					671	539
Strategic Growth Initiative Plan
Restructuring Cost and Reserve [Line Items]
Net restructuring charges recognized	50				161
Restructuring liabilities	80				80		$ 80	$ 0
Payments for severance					56
Strategic Growth Initiative Plan | Other accrued liabilities
Restructuring Cost and Reserve [Line Items]
Restructuring liabilities	49				49		49
Strategic Growth Initiative Plan | Other noncurrent liabilities
Restructuring Cost and Reserve [Line Items]
Restructuring liabilities	31				31		31
Strategic Growth Initiative Plan | Employee severance, exit-related costs and asset impairment charges
Restructuring Cost and Reserve [Line Items]
Net restructuring charges recognized	19				130
Net restructuring charges recognized, after tax	14				114
Strategic Growth Initiative Plan | Minimum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	150				150		150
Strategic Growth Initiative Plan | Maximum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	210				210		210
Strategic Growth Initiative Plan - Relocation of Corporate Headquarters | Severance
Restructuring Cost and Reserve [Line Items]
Net restructuring charges recognized	31				31
Net restructuring charges recognized, after tax	23				23
Strategic Growth Initiative Plan - Relocation of Corporate Headquarters | Minimum | Severance
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	60				60		60
Strategic Growth Initiative Plan - Relocation of Corporate Headquarters | Maximum | Severance
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	120				120		120
Other | Severance and other restructuring-related costs
Restructuring Cost and Reserve [Line Items]
Net restructuring charges recognized	21				54
Net restructuring charges recognized, after tax	16				40
Fiscal 2018 McKesson Europe Plan
Restructuring Cost and Reserve [Line Items]
Pre-tax restructuring and asset impairment charges				$ 189
After-tax restructuring and asset impairment charges				$ 157
Restructuring liabilities	24				24		24	42
Pre-tax charges incurred to-date							89
Fiscal 2018 McKesson Europe Plan | Severance
Restructuring Cost and Reserve [Line Items]
Net restructuring charges recognized			6			53
Net restructuring charges recognized, after tax			5			45
Payments for severance	10				26
Fiscal 2018 McKesson Europe Plan | Severance and lease exit costs
Restructuring Cost and Reserve [Line Items]
Net restructuring charges recognized	4				15
Net restructuring charges recognized, after tax	4				13
Fiscal 2018 McKesson Europe Plan | Minimum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	90				90		90
Fiscal 2018 McKesson Europe Plan | Maximum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	130				130		130
Fiscal 2016 Cost Alignment Plan
Restructuring Cost and Reserve [Line Items]
Payments for severance	3		$ 9		14	32
Fiscal 2016 Cost Alignment Plan | Other accrued liabilities
Restructuring Cost and Reserve [Line Items]
Restructuring liabilities	22				22		22	39
Fiscal 2016 Cost Alignment Plan | Other noncurrent liabilities
Restructuring Cost and Reserve [Line Items]
Restructuring liabilities	27				$ 27		$ 27	$ 30
Operating Segments | European Pharmaceutical Solutions
Restructuring Cost and Reserve [Line Items]
Pre-tax restructuring and asset impairment charges						242
After-tax restructuring and asset impairment charges						$ 202
Rexall Health
Restructuring Cost and Reserve [Line Items]
After-tax impairment charges	35
Impairment of long-lived and intangible assets	$ 35