Segment Information - Assets Allocated by Segment (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Cash and cash equivalents	$ 223.0	$ 318.8
Restricted cash	0.2	16.2
Real estate	1,049.5	498.0
Investment in unconsolidated ventures	7.3	11.7
Income taxes receivable	3.3	4.4
Property and equipment, net	2.4	1.7
Deferred tax asset, net	21.0	26.9
Other assets	18.5	15.4
Total Assets	$ 1,325.2	893.1
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Cash and cash equivalents		318.8
Restricted cash		16.2
Real estate		0.0
Investment in unconsolidated ventures		0.0
Income taxes receivable		4.4
Property and equipment, net		0.2
Deferred tax asset, net		26.9
Other assets		2.6
Total Assets		369.1
Real Estate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Cash and cash equivalents		0.0
Restricted cash		0.0
Real estate		498.0
Investment in unconsolidated ventures		11.7
Income taxes receivable		0.0
Property and equipment, net		0.0
Deferred tax asset, net		0.0
Other assets		12.4
Total Assets		522.1
Other [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Cash and cash equivalents		0.0
Restricted cash		0.0
Real estate		0.0
Investment in unconsolidated ventures		0.0
Income taxes receivable		0.0
Property and equipment, net		1.5
Deferred tax asset, net		0.0
Other assets		0.4
Total Assets		$ 1.9