Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Income Tax Contingency [Line Items]
Income tax benefit related to release of valuation allowance		$ 33
Income tax expense (benefit)	$ (12)		$ 48	$ (7)	$ 98
U.S. federal corporate statutory tax rate	21.00%			21.00%
Reasonably possible decrease in gross unrecognized tax benefits from ongoing audits and settlement	$ 1			$ 1
Foreign Tax Authority
Income Tax Contingency [Line Items]
Release of valuation allowance						$ 33