Quarterly Financial Information (Unaudited) - Narrative (Details) $ in Millions		3 Months Ended								12 Months Ended
	Mar. 09, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($) reporting_unit	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($) reporting_unit	Mar. 31, 2018 USD ($)
Quarterly Financial Information [Line Items]
Pretax credits related to last-in-first-out method of accounting for inventory		$ 138	$ 66	$ 33	$ 15	$ 146	$ 21	$ 22	$ 21	$ 252	$ 210	$ 99
Pretax credits related to last-in-first-out method of accounting for inventory, after tax		101	49	25	11	109	15	17	15	186	156	64
Estimated net gain on split-off of the Change Healthcare JV										0	56	43
Long-lived asset impairment charges, before tax						190			20	30
Long-lived asset impairment charges, net of tax						156			16	30
Proportionate share of income (loss) from Change Healthcare										36	43	32
Net cash proceeds from settlements						63	104		35
Goodwill impairment charge										2	$ 1,797	1,738
Number of reporting units | reporting_unit											2
Non-cash after-tax charge												1,700
Rexall Health
Quarterly Financial Information [Line Items]
Long-lived asset impairment charges, before tax			32
Long-lived asset impairment charges, net of tax			32
CANADA | Rexall Health | Third Party Seller of Rexall Health
Quarterly Financial Information [Line Items]
Proceeds from escrow settlement related to previous Acquisition									97		$ 97
Proceeds from escrow settlement related to previous Acquisition, after tax									97		97
Rexall Health
Quarterly Financial Information [Line Items]
Long-lived asset impairment charges, before tax							35					488
Long-lived asset impairment charges, net of tax												488
European Pharmaceutical Solutions
Quarterly Financial Information [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
Goodwill impairment charge						1,200			$ 570	0	1,776	1,300
Number of reporting units | reporting_unit									2
Non-cash after-tax charge						1,200			$ 570		1,800	1,300
For Two Counties In re: National Prescription Opiate Litigation
Quarterly Financial Information [Line Items]
Litigation settlement charges				82						82
Litigation settlement charges, after tax				61						61
United States | Pension Plans, Defined Benefit
Quarterly Financial Information [Line Items]
Pre-tax pension settlement charges				105	17					122
After-tax pension settlement charges				78	12					90
German Wholesale Joint Venture | Held-for-sale
Quarterly Financial Information [Line Items]
Pre-tax impairment charge			282
After-tax impairment charge			282
Corporate Joint Venture
Quarterly Financial Information [Line Items]
Pre-tax dilution loss				246						246
After-tax dilution loss				184						184
Change Healthcare
Quarterly Financial Information [Line Items]
Estimated net gain on split-off of the Change Healthcare JV	$ 414
Proportionate share of income (loss) from Change Healthcare		$ (44)	$ (28)	(51)	$ 4	$ (32)	$ (50)	(56)	$ (56)
Change Healthcare | Corporate Joint Venture
Quarterly Financial Information [Line Items]
Pre-tax other-than-temporary impairment				1,200						1,200
After-tax other-than-temporary impairment				$ 864						864
Proportionate share of income (loss) from Change Healthcare										$ (119)	(194)	$ (248)
Pre-tax credit representing reduction in TRA liability								90			90
After-tax credit representing reduction in TRA liability								$ 66			$ 66