Segment Information - Assets Allocated by Segment (Detail) - USD ($)	Dec. 31, 2018	Sep. 30, 2018
Segment Reporting Information [Line Items]
Cash and cash equivalents	$ 154,200,000	$ 318,800,000
Restricted cash	16,100,000	16,200,000
Real estate	693,200,000	498,000,000
Investment in unconsolidated ventures	7,400,000	11,700,000
Income taxes receivable	4,500,000	4,400,000
Property and equipment, net	1,800,000	1,700,000
Deferred tax asset, net	25,500,000	26,900,000
Other assets	16,000,000	15,400,000
Total Assets	$ 918,700,000	893,100,000
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Cash and cash equivalents		318,800,000
Restricted cash		16,200,000
Real estate		0
Investment in unconsolidated ventures		0
Income taxes receivable		4,400,000
Property and equipment, net		200,000
Deferred tax asset, net		26,900,000
Other assets		2,600,000
Total Assets		369,100,000
Real Estate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Cash and cash equivalents		0
Restricted cash		0
Real estate		498,000,000
Investment in unconsolidated ventures		11,700,000
Income taxes receivable		0
Property and equipment, net		0
Deferred tax asset, net		0
Other assets		12,400,000
Total Assets		522,100,000
Other [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Cash and cash equivalents		0
Restricted cash		0
Real estate		0
Investment in unconsolidated ventures		0
Income taxes receivable		0
Property and equipment, net		1,500,000
Deferred tax asset, net		0
Other assets		400,000
Total Assets		$ 1,900,000