Income Taxes - Summary of Federal and State Jurisdiction Tax (Detail) - USD ($)	6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2014
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	$ 22	$ 19	$ 17	$ 16	$ 11	$ 9	$ 1
United States [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0	0	0
California [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0	0	$ 0
COLORADO
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0			0
CONNECTICUT
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0
DISTRICT OF COLUMBIA
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0
FLORIDA
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0
GEORGIA
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0
ILLINOIS
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0	0
INDIANA
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0
IOWA
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0
LOUISIANA
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0
MARYLAND
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0	0
Massachusetts [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0	0
MICHIGAN
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0	0
MINNESOTA
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0	0
NEW HAMPSHIRE
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0	0
NEW JERSEY
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0
NEW YORK
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0
North Carolina [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0	0
OREGON
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0
PENNSYLVANIA
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0
Texas [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	$ 0	$ 0
WISCONSIN
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	$ 0	$ 0	$ 0	$ 0