Goodwill and Intangible Assets, Net - Narrative - Intangible Assets (Details)	3 Months Ended			6 Months Ended	9 Months Ended
	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) segment	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2020 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Number of reportable segments | segment		4		4
Goodwill impairment charge	$ 0		$ 2,000,000		$ 69,000,000	$ 2,000,000
Goodwill	9,511,000,000			$ 9,511,000,000	9,511,000,000		$ 9,360,000,000
Amortization expense of intangible assets	108,000,000		$ 113,000,000		320,000,000	$ 343,000,000
Finite-Lived Intangible Assets, Amortization Expense, Maturity Schedule [Abstract]
Estimated annual amortization expense, remainder of 2021	96,000,000			96,000,000	96,000,000
Estimated annual amortization expense, 2022	371,000,000			371,000,000	371,000,000
Estimated annual amortization expense, 2023	272,000,000			272,000,000	272,000,000
Estimated annual amortization expense, 2024	254,000,000			254,000,000	254,000,000
Estimated annual amortization expense, 2025	251,000,000			251,000,000	251,000,000
Estimated annual amortization expense, thereafter	1,700,000,000			1,700,000,000	1,700,000,000
International
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Goodwill impairment charge		$ 69,000,000			69,000,000
After-tax goodwill impairment		$ 69,000,000			69,000,000
Goodwill	1,526,000,000			1,526,000,000	1,526,000,000		$ 1,443,000,000
Retail Pharmacy Reporting Unit | International | Europe
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Goodwill	$ 0			$ 0	$ 0