Investment in Unconsolidated Ventures - Summarized Balance Sheet Information (Details) - USD ($) $ in Millions	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 355.6			$ 382.8	$ 223.2			$ 335.0
Real estate	1,287.1			1,028.9
Other assets	27.1			13.7
Assets	1,685.4			1,455.7
Liabilities	838.5			646.8
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	846.9	$ 836.8	$ 825.6	808.9	$ 695.3	$ 687.8	$ 678.0	$ 674.5
Liabilities and Equity	1,685.4			1,455.7
Equity Method Investment, Nonconsolidated Investee or Group of Investees
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	1.2			1.6
Real estate	9.4			13.6
Other assets	0.2			0.1
Assets	10.8			15.3
Liabilities	0.2			0.3
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	10.6			15.0
Liabilities and Equity	$ 10.8			$ 15.3