Summary of Significant Accounting Policies (Income Taxes) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Deferred Tax Assets, Net			$ 1,902	$ 33
Deferred Tax Assets, Valuation Allowance			288	162
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	$ 630	$ 1,170	1,760
Income Tax Expense (Benefit)			(1,531)	$ 60	$ 406
Swiss Federal Tax Administration (FTA) [Member]
Deferred Tax Assets, Net			1,840
Deferred Tax Assets, Valuation Allowance			131
Reduction in Taxes [Member]
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability			393
Income Tax Expense (Benefit)			$ 80