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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, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024	Sep. 30, 2024
Real Estate Properties [Line Items]
Real estate	$ 2,759.4		$ 2,759.4		$ 2,266.2
Asset Impairment Charges			0.0	$ 0.0
Loss on Contract Termination	0.9	$ 0.2	2.0	$ 0.4
Payments to Develop Real Estate Assets			564.2
Payments to Acquire Residential Real Estate			460.4
Developed and under development projects
Real Estate Properties [Line Items]
Real estate	2,506.6		2,506.6		2,126.1
Land held for future development
Real Estate Properties [Line Items]
Real estate	$ 252.8		$ 252.8		$ 140.1
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%