Income Taxes - Summary of Federal and State Jurisdiction Tax (Detail) - USD ($)	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
United States [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	$ 0	$ 0	$ 0	$ 0	$ 0
California
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0
Connecticut [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0
District Of Columbia [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0
Florida [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0
Georgia [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0
Illinois [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0
Louisiana [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0
Maryland [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0
Massachusetts [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0
Michigan [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0
Minnesota [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0
New Hampshire [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0
New York [Member]
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
Oregon [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0
Texas [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	0	0	0	0	0
Wisconsin [Member]
Investments, Owned, Federal Income Tax Note [Line Items]
Tax Jurisdiction	$ 0	$ 0	$ 0	$ 0	$ 0