Commitments and Contingencies (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2020	Sep. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
Letters of Credit Outstanding, Amount		$ 36.0
Special Assessment Bond		236.9
Operating Leases, Rent Expense	$ 0.6	1.1	$ 0.7
Lessee, Operating Lease, Liability, to be Paid, Year One		1.2
Lessee, Operating Lease, Liability, to be Paid, Year Two		0.9
Lessee, Operating Lease, Liability, to be Paid, Year Three		0.9
Lessee, Operating Lease, Liability, to be Paid, Year Four		0.7
Lessee, Operating Lease, Liability, to be Paid, Year Five		0.3
Lessee, Operating Lease, Liability, to be Paid, after Year Five		$ 0.0