Income Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Income Tax Contingency [Line Items]
Income tax expense (benefit)				$ 18	$ 356	$ (53)
Income tax rates (percent)				1.60%	58.40%	(22.20%)
Statutory federal income tax rate (percent)				21.00%	21.00%	31.60%
Research and development credit				$ 7	$ 7	$ 11
Gain from sale of equity method investment, pre-tax				0	56	43
Non-cash pre-tax charge				2	1,797	1,738
Non-cash after-tax charge						1,700
Net discrete tax benefit		$ 42
Net tax benefit on intellectual property transfer				0	42	178
Valuation allowance	$ 870			833	870
Increase (decrease) in valuation allowance				(37)
Unrecognized tax benefits	1,052			958	1,052	1,183	$ 486
Unrecognized tax benefits that would Impact income tax expense and the effective tax rate				763
Decrease in unrecognized tax benefits				91	171
Income tax expense (benefit), before any tax effect, related to accrued interest and penalties				23	33	(1)
Accrued interest and penalties on unrecognized tax benefits	68			91	68
Undistributed earnings of foreign operations				5,000
Federal
Income Tax Contingency [Line Items]
Federal, state and foreign net operating loss carryforwards				75
State
Income Tax Contingency [Line Items]
Federal, state and foreign net operating loss carryforwards				3,300
Foreign Tax Authority
Income Tax Contingency [Line Items]
Federal, state and foreign net operating loss carryforwards				1,900
European Pharmaceutical Solutions
Income Tax Contingency [Line Items]
Charge for remeasurement to fair value				275
Charge for remeasurement to fair value, after tax				275
Non-cash pre-tax charge	1,200		$ 570	0	1,776	1,300
Non-cash after-tax charge	$ 1,200		$ 570		$ 1,800	$ 1,300
Capital Loss Carryforward | Foreign Tax Authority
Income Tax Contingency [Line Items]
Federal, state and foreign net operating loss carryforwards				739
Expense Related to Foreign Losses
Income Tax Contingency [Line Items]
Increase (decrease) in valuation allowance				(30)
Remeasurement of Foreign Loss Carryforwards
Income Tax Contingency [Line Items]
Increase (decrease) in valuation allowance				$ (67)