Segments of Business - Narrative (Details) $ in Millions		3 Months Ended										12 Months Ended
	Mar. 09, 2020 USD ($)	Mar. 31, 2020 USD ($) country	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 ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2020 USD ($) segment country	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment												3
Credits associated with last-in, first-out inventory method		$ 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
Net cash proceeds from settlements						63	104		35
Pre-tax charges related to customer bankruptcy													61
After-tax charges related to customer bankruptcy													45
Gain from sale of equity method investment, pre-tax												0	56	43
Long-lived asset impairment charges, before tax						190			20			30
Long-lived asset impairment charges, net of tax						156			16			30
Goodwill impairment charge												2	1,797	1,738
Non-cash after-tax charge														1,700
Equity earnings and charges from investment in Change Healthcare Joint Venture												(36)	(43)	(32)
Pre-tax gain on sale of business												0	0	37
Employee Severance and Other Exit Related Costs
Segment Reporting Information [Line Items]
Pre-tax charge													94
Restructuring charges, after tax													70
Held-for-sale | Enterprise Information Solutions
Segment Reporting Information [Line Items]
Pre-tax gain on sale of business											$ 109			109
Gain from sale of business, after tax											$ 30			30
For Two Counties In re: National Prescription Opiate Litigation
Segment Reporting Information [Line Items]
Litigation settlement charges				82								82
Litigation settlement charges, after tax				61								61
United States | Pension Plans, Defined Benefit
Segment Reporting Information [Line Items]
Pre-tax pension settlement charges				105	17							122
After-tax pension settlement charges				78	12							90
Rexall Health
Segment Reporting Information [Line Items]
Long-lived asset impairment charges, before tax			32
Long-lived asset impairment charges, net of tax			32
Rexall Health | Third Party Seller of Rexall Health | CANADA
Segment Reporting 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
Segment Reporting Information [Line Items]
Long-lived asset impairment charges, before tax							35							488
Long-lived asset impairment charges, net of tax														488
Change Healthcare
Segment Reporting Information [Line Items]
Gain from sale of equity method investment, pre-tax	$ 414
Equity earnings and charges from investment in Change Healthcare Joint Venture		$ 44	28	51	$ (4)	32	50	56	56
Corporate Joint Venture
Segment Reporting Information [Line Items]
Pre-tax dilution loss				246								246
After-tax dilution loss				184								184
Corporate Joint Venture | Change Healthcare
Segment Reporting Information [Line Items]
Pre-tax other-than-temporary impairment				1,200								1,200
After-tax other-than-temporary impairment				$ 864								864
Pre-tax credit representing reduction in TRA liability								90					90
After-tax credit representing reduction in TRA liability								$ 66					66
Equity earnings and charges from investment in Change Healthcare Joint Venture												119	194	248
Customer relationships | Rexall Health | Intangible Asset and Store Assets Impairment
Segment Reporting Information [Line Items]
Long-lived asset impairment charges, before tax												32	56
Long-lived asset impairment charges, net of tax												32	56
U.S. Pharmaceutical and Specialty Solutions
Segment Reporting Information [Line Items]
Gain from sale of equity method investment, pre-tax														43
Gain from sale of equity method investment, after tax														26
Goodwill impairment charge												0	0
U.S. Pharmaceutical and Specialty Solutions | Operating Segments
Segment Reporting Information [Line Items]
Net cash proceeds from settlements												$ 22	202	144
European Pharmaceutical Solutions
Segment Reporting Information [Line Items]
Number of countries in which entity operates | country		13										13
Charge for remeasurement to fair value												$ 275
Charge for remeasurement to fair value, after tax												275
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
Non-cash after-tax charge						$ 1,200			$ 570				1,800	1,300
Medical-Surgical Solutions
Segment Reporting Information [Line Items]
Goodwill impairment charge												0	0
Other
Segment Reporting Information [Line Items]
Gain from sale of equity method investment, pre-tax													56
Gain from sale of equity method investment, after tax													41
Goodwill impairment charge												2	21
Pre-tax charge													91
Restructuring charges, after tax													$ 86
Other | Operating Segments
Segment Reporting Information [Line Items]
Pre-tax credit representing reduction in TRA liability														46
After-tax credit representing reduction in TRA liability														$ 30
Other | Rexall Health
Segment Reporting Information [Line Items]
Goodwill impairment charge							$ 21			$ 455
Other | Change Healthcare
Segment Reporting Information [Line Items]
Gain from sale of equity method investment, pre-tax		$ 414										414
Gain from sale of equity method investment, after tax		$ 414										$ 414
Services | U.S. Pharmaceutical and Specialty Solutions | Product Concentration Risk | Revenues
Segment Reporting Information [Line Items]
Percentage of total revenue (less than) (as a percent)												1.00%	1.00%	1.00%
Services | European Pharmaceutical Solutions | Product Concentration Risk | Revenues
Segment Reporting Information [Line Items]
Percentage of total revenue (less than) (as a percent)												10.00%	10.00%	10.00%
Services | Medical-Surgical Solutions | Product Concentration Risk | Revenues
Segment Reporting Information [Line Items]
Percentage of total revenue (less than) (as a percent)												2.00%	2.00%	2.00%