Income Taxes - Effective Tax Rate Reconciliation, Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Income Tax Rate Reconciliation [Line Items]
Effective income tax rate reconciliation, change in deferred tax assets valuation allowance, amount	$ 256
Change in valuation allowances	16.90%	(7.70%)	(34.30%)
Effective income tax rate reconciliation, change in deferred revenue accounting method, amount	$ 66
Effective income tax rate reconciliation, change in deferred revenue accounting method, percent	4.30%
Effective income tax rate reconciliation, foreign subsidiary basis, amount		$ (554)
Effective income tax rate reconciliation, foreign subsidiary basis, percentage		42.50%
Effective income tax rate reconciliation, Tax Cuts and Jobs Act of 2017, transition tax on accumulated foreign earnings, amount		$ 361
Effective income tax rate reconciliation, Tax Cuts and Jobs Act of 2017, transition tax on accumulated foreign earnings, percent	(3.90%)	27.70%
Effective income tax rate reconciliation, remeasurement of deferred tax assets and liabilities, amount		$ 338
Effective income tax rate reconciliation, remeasurement of deferred tax assets and liabilities, percent		25.90%
Effective income tax rate reconciliation, nondeductible transaction costs, amount			$ 21
Effective income tax rate reconciliation, nondeductible transaction costs, percent	0.10%	1.00%	(12.10%)
Effective income tax rate reconciliation, change in accounting policy, amount			$ 20
Excess tax benefits for stock compensation	(1.10%)	(3.00%)	11.30%
Luxembourg
Income Tax Rate Reconciliation [Line Items]
Effective income tax rate reconciliation, gain (loss) on investments, amount	$ 360
Effective income tax rate reconciliation, gain (loss) on investments, percent	23.70%
U.S., Germany and Luxembourg | Foreign
Income Tax Rate Reconciliation [Line Items]
Effective income tax rate reconciliation, change in deferred tax assets valuation allowance, amount			$ 135
Change in valuation allowances			78.00%
Denmark, Japan and U.K. | Foreign
Income Tax Rate Reconciliation [Line Items]
Effective income tax rate reconciliation, change in deferred tax assets valuation allowance, amount			$ (75)
Change in valuation allowances			(43.00%)