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Real Estate - Text (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
Real Estate - Text (Detail) - USD ($) $ in Millions	Mar. 31, 2026	Mar. 31, 2025	Mar. 31, 2026	Mar. 31, 2025	Sep. 30, 2025
Real Estate Properties [Line Items]
Real estate	$ 2,709.7		$ 2,709.7		$ 2,645.1
Payments to Acquire Residential Real Estate			156.2
Payments to Develop Real Estate Assets			537.8
Asset, Impairment Loss			0.0	$ 0.0
Loss on Contract Termination	6.3	$ 0.9	7.1	$ 2.0
Developed and under development projects
Real Estate Properties [Line Items]
Real estate	2,552.7		2,552.7		2,465.1
Land held for future development
Real Estate Properties [Line Items]
Real estate	$ 157.0		$ 157.0		$ 180.0
D.R. Horton, Inc. [Member] \| Minimum
Real Estate Properties [Line Items]
Related Party Transaction, Rate			12.00%
D.R. Horton, Inc. [Member] \| Maximum
Real Estate Properties [Line Items]
Related Party Transaction, Rate			16.00%