Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018
Income Tax Contingency [Line Items]
Income tax (expense) benefit	$ 50,000,000	$ 54,000,000
Income (loss) before income tax (expense) benefit	178,000,000	141,000,000
U.S. statutory federal tax rate			21.00%
Gross income tax paid	29,000,000	48,000,000
Income tax refunds	$ 10,000,000	$ 0
U.S. Tax Cuts and Jobs Act [Member]
Income Tax Contingency [Line Items]
Income tax (expense) benefit			$ 237,000,000