Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended					6 Months Ended		12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Tax Contingency [Line Items]
U.S. federal corporate statutory tax rate		21.00%		24.50%	35.00%	21.00%	24.50%		24.50%
Enacted tax rate						21.00%
Tax Act, income tax expense			$ 23,000,000
Income tax expense	$ 48,000,000			$ 19,000,000		$ 98,000,000	$ 71,000,000
Income withholding taxes and foreign losses				$ 0			0
Tax benefit due to reversal of deferred tax assets valuation allowance							$ 26,000,000
Maximum
Income Tax Contingency [Line Items]
Reasonably possible decrease in gross unrecognized tax benefits from ongoing audits and settlement	$ 2,000,000					$ 2,000,000
Scenario, Plan
Income Tax Contingency [Line Items]
U.S. federal corporate statutory tax rate								21.00%
Scenario, Forecast
Income Tax Contingency [Line Items]
Income tax liability related to Transition Tax								$ 0