Restructuring, Impairment and Related Charges - Narrative (Details) - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018
Restructuring Cost and Reserve [Line Items]
Restructuring and asset impairment charges					$ 268	$ 597	$ 567
Long-lived asset impairment charges, before tax		$ 190		$ 20	30
Long-lived asset impairment charges, net of tax		156		$ 16	30
Rexall Health
Restructuring Cost and Reserve [Line Items]
Long-lived asset impairment charges, before tax			$ 35				488
Long-lived asset impairment charges, net of tax							488
European Pharmaceutical Solutions
Restructuring Cost and Reserve [Line Items]
Long-lived asset impairment charges, before tax	$ 64				82	210	446
Long-lived asset impairment charges, net of tax	$ 53				66	172	410
Intangible Asset and Store Assets Impairment | Rexall Health | Customer relationships
Restructuring Cost and Reserve [Line Items]
Long-lived asset impairment charges, before tax					32	56
Long-lived asset impairment charges, net of tax					32	56
Intangible asset impairment charges, before tax						35	33
Intangible asset impairment charges, net of tax						35	33
Strategic Growth Initiative Plan
Restructuring Cost and Reserve [Line Items]
Pre-tax charge					131	331
Cash payments for restructuring					136
Reserve balance		150			110	150
Strategic Growth Initiative Plan | Employee Severance, Exit-related Costs and Asset Impairment Charges
Restructuring Cost and Reserve [Line Items]
Pre-tax charge					15	135
Restructuring charges, after tax					12	122
Strategic Growth Initiative Plan - Relocation of Corporate Headquarters
Restructuring Cost and Reserve [Line Items]
Restructuring incurred to-date					77
Strategic Growth Initiative Plan - Relocation of Corporate Headquarters | Minimum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges					80
Strategic Growth Initiative Plan - Relocation of Corporate Headquarters | Maximum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges					130
Strategic Growth Initiative Plan - Relocation of Corporate Headquarters | Employee Retention Expenses, Asset Impairments and Accelerated Depreciation
Restructuring Cost and Reserve [Line Items]
Pre-tax charge					44	33
Restructuring charges, after tax					32	24
Strategic Growth Initiative Plan - Additional Global Reorganization and Business Consolidation Programs | Minimum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges					300
Strategic Growth Initiative Plan - Additional Global Reorganization and Business Consolidation Programs | Maximum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges					350
Strategic Growth Initiative Plan - Additional Global Reorganization and Business Consolidation Programs | Employee Severance, Accelerated Depreciation and Project Consulting Fees
Restructuring Cost and Reserve [Line Items]
Pre-tax charge					72	163
Restructuring charges, after tax					55	127
Fiscal 2018 McKesson Europe Plan | Minimum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges								$ 90
Fiscal 2018 McKesson Europe Plan | Maximum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges								$ 130
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			4	19
Fiscal 2018 McKesson Europe Plan | Employee Severance
Restructuring Cost and Reserve [Line Items]
Cash payments for restructuring						32	$ 10
Other Restructuring Plans
Restructuring Cost and Reserve [Line Items]
Reserve balance		$ 68			$ 43	$ 68