Restructuring, Impairment, and Related Charges - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019	May 12, 2021
Restructuring Cost and Reserve [Line Items]
Restructuring and asset impairment charges	$ 334	$ 268	$ 597
Pre-tax charge	219	156	352
Reserve balance	151	157	237
International
Restructuring Cost and Reserve [Line Items]
Long-lived asset impairment charges, before tax	115	112	245
International | Retail Pharmacy
Restructuring Cost and Reserve [Line Items]
Long-lived asset impairment charges, before tax		82	210
International | Rexall Health
Restructuring Cost and Reserve [Line Items]
Long-lived asset impairment charges, before tax		30
International | U.K.
Restructuring Cost and Reserve [Line Items]
Pre-tax charge	57
Minimum | International | U.K.
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	85
Maximum | International | U.K.
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges	90
Employee Severance, Exit-related Costs and Asset Impairment Charges
Restructuring Cost and Reserve [Line Items]
Pre-tax charge		15	135
Intangible Asset and Store Assets Impairment | Rexall Health | Customer relationships
Restructuring Cost and Reserve [Line Items]
Intangible asset impairment charges, before tax			35
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	62	72	163
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	$ 28	$ 44	33
Operating Model and Cost Optimization Programs | Employee Severance, Exit-related Costs and Asset Impairment Charges
Restructuring Cost and Reserve [Line Items]
Pre-tax charge			18
Restructuring incurred to-date			$ 92
Subsequent Event | Minimum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges				$ 180
Subsequent Event | Maximum
Restructuring Cost and Reserve [Line Items]
Expected total pre-tax charges				$ 280