Restructuring and Asset Impairment Charges - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Restructuring Cost and Reserve [Line Items]
Restructuring and asset impairment charges			$ 597	$ 567	$ 18
Intangible Asset and Store Assets Impairment | Customer relationships
Restructuring Cost and Reserve [Line Items]
Intangible asset impairment charges, before tax			35	33
Intangible asset impairment charges, net of tax		$ 33	35
Strategic Growth Initiative Plan
Restructuring Cost and Reserve [Line Items]
Pre-tax charge			331
Cash payments for restructuring			141
Reserve balance	$ 150	0	150	0
Strategic Growth Initiative Plan | Other Accrued Liabilities
Restructuring Cost and Reserve [Line Items]
Reserve balance	117		117
Strategic Growth Initiative Plan | Other Noncurrent Liabilities
Restructuring Cost and Reserve [Line Items]
Reserve balance	33		33
Strategic Growth Initiative Plan | Employee Severance, Exit-related Costs and Asset Impairment Charges
Restructuring Cost and Reserve [Line Items]
Pre-tax charge			135
Restructuring charges, after tax			122
Strategic Growth Initiative Plan | Minimum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	140		140
Strategic Growth Initiative Plan | Maximum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	180		180
Strategic Growth Initiative Plan - Relocation of Corporate Headquarters | Reduction in Workforce
Restructuring Cost and Reserve [Line Items]
Pre-tax charge			33
Restructuring charges, after tax			24
Strategic Growth Initiative Plan - Relocation of Corporate Headquarters | Minimum | Reduction in Workforce
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	80		80
Strategic Growth Initiative Plan - Relocation of Corporate Headquarters | Maximum | Reduction in Workforce
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	130		130
Strategic Growth Initiative Plan - Additional Global Reorganization and Business Consolidation Programs | Reduction in Workforce
Restructuring Cost and Reserve [Line Items]
Pre-tax charge	163
Restructuring charges, after tax	127
Strategic Growth Initiative Plan - Additional Global Reorganization and Business Consolidation Programs | Minimum | Reduction in Workforce
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	300		300
Strategic Growth Initiative Plan - Additional Global Reorganization and Business Consolidation Programs | Maximum | Reduction in Workforce
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	350		350
Cost Alignment Plan
Restructuring Cost and Reserve [Line Items]
Restructuring incurred to-date	256		256
Cost Alignment Plan | Other Accrued Liabilities
Restructuring Cost and Reserve [Line Items]
Reserve balance	9	39	9	39
Cost Alignment Plan | Other Noncurrent Liabilities
Restructuring Cost and Reserve [Line Items]
Reserve balance	25	30	25	30
Cost Alignment Plan | Reduction in Workforce
Restructuring Cost and Reserve [Line Items]
Cash payments for restructuring			18	45
Cost Alignment Plan | Minimum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	250		250
Cost Alignment Plan | Maximum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	270		270
Fiscal 2018 McKesson Europe Plan
Restructuring Cost and Reserve [Line Items]
Restructuring and asset impairment charges			210	446
Restructuring and asset impairment charges, net of tax			172	410
Fiscal 2018 McKesson Europe Plan | Reduction in Workforce
Restructuring Cost and Reserve [Line Items]
Cash payments for restructuring			32	10
Fiscal 2018 McKesson Europe Plan | Severance and Lease Exit Costs
Restructuring Cost and Reserve [Line Items]
Pre-tax charge			18	74
Restructuring charges, after tax			16	67
Restructuring incurred to-date	92		92
Reserve balance	19	$ 42	19	$ 42
Fiscal 2018 McKesson Europe Plan | Minimum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	90		90
Fiscal 2018 McKesson Europe Plan | Maximum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	$ 130		$ 130