Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020
Income Taxes, Additional Information [Line Items]
Effective income tax rate	34.00%	5.00%		26.00%	15.00%
Ireland statutory income tax rate				12.50%	12.50%
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount	$ 105
Gross tax effected unrecognized tax benefits						$ 2,528
Amount of unrecognized tax benefits which may impact effective tax rate						2,132
Total net accrued interest, net of tax benefit						205
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense				$ 24	$ 35
Unrecognized Tax Benefit Decrease Resulting From Tax Audit Resolution		$ 22
Mark-to-market gain (loss)	207
Mark-to-market gain (loss), tax	53
Restructuring and impairment costs	96	62		$ 96	$ 173
Restructuring charges, tax	15	$ 4	$ 16
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount			30
2020 Restructuring Plan
Income Taxes, Additional Information [Line Items]
Restructuring and impairment costs			$ 111			$ 297
2021 Restructuring Plan
Income Taxes, Additional Information [Line Items]
Restructuring and impairment costs	$ 96