Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended			6 Months Ended		9 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Tax Contingency [Line Items]
U.S. corporate statutory tax rate		35.00%	35.00%		35.00%
Enacted tax rate	21.00%
Tax Act, provisional income tax expense		$ 26,000,000		$ 26,000,000
Tax Act, provisional deferred tax liability				$ 4,000,000
One-time transition tax on accumulated foreign earnings, tax rate for cash and cash equivalents				15.50%
One-time transition tax on accumulated foreign earnings, tax rate for illiquid assets				8.00%
Income tax liability related to Transition Tax	$ 0			$ 0
Income tax expense	19,000,000		$ 3,000,000	71,000,000	$ 20,000,000
Income withholding taxes and foreign losses uncertain tax positions	0		$ 0	0	$ 0
Maximum
Income Tax Contingency [Line Items]
Reasonably possible decrease in gross unrecognized tax benefits from ongoing audits and settlement	$ 3,000,000			$ 3,000,000
Scenario, Plan
Income Tax Contingency [Line Items]
U.S. corporate statutory tax rate						21.00%	21.00%	24.50%