Investment in Unconsolidated Ventures (Details) $ in Millions	3 Months Ended								9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) venture	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2020 USD ($) venture	Sep. 30, 2019 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 394.3				$ 382.8						$ 394.3	$ 382.8
Real estate	1,309.7				1,028.9						1,309.7	1,028.9
Other assets	24.9				13.7						24.9	13.7
Total assets	1,739.9				1,455.7						1,739.9	1,455.7
Accounts payable and other liabilities	868.1				646.8						868.1	646.8
Equity	871.8				808.9				$ 674.5		871.8	808.9	$ 605.6
Total liabilities and equity	1,739.9				1,455.7						1,739.9	1,455.7
Investment in unconsolidated ventures	3.6				7.3						3.6	7.3
Revenues	347.6	$ 177.9	$ 159.1	$ 247.2	236.3	$ 88.2	$ 65.3	$ 38.5	78.3	$ 83.5	931.8	428.3
Net income	$ 24.5	$ 10.1	$ 10.4	$ 16.8	12.6	$ 6.9	$ 12.8	$ 3.9	70.0	67.9	61.7	36.3
Income (Loss) from Equity Method Investments									4.8	10.9	0.7	0.5
Return of investment in unconsolidated ventures									0.8		4.3	0.1
Gain on sale of assets									27.8	$ 113.4	$ 0.1	3.0
Number of Equity Method Investments | venture	4										4
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 1.2				1.6						$ 1.2	1.6
Real estate	6.1				13.6						6.1	13.6
Other assets	0.2				0.1						0.2	0.1
Total assets	7.5				15.3						7.5	15.3
Accounts payable and other liabilities	0.2				0.3						0.2	0.3
Equity	7.3				15.0						7.3	15.0
Total liabilities and equity	$ 7.5				$ 15.3						7.5	15.3
Revenues									22.2		3.5	1.9
Net income									15.1		3.8	1.3
Payments to Acquire Equity Method Investments											0.0
Return of investment in unconsolidated ventures									$ 4.3		$ 4.3	$ 5.0
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member] | Starwood Land, L.P.. [Member]
Schedule of Equity Method Investments [Line Items]
Number of Equity Method Investments Sold									8
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member] | Lm Land Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Total assets									$ 21.6
Accounts payable and other liabilities									0.4
Equity									21.2
Investment in unconsolidated ventures									8.9
Revenues									17.4
Income (Loss) from Equity Method Investments									6.4
Income (Loss) from Continuing Operations before Income Taxes, Domestic									18.1
Deferred Revenue, Revenue Recognized									5.7
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member] | FOR/SR Forsyth LLC [Member]
Schedule of Equity Method Investments [Line Items]
Proceeds from Divestiture of Businesses and Interests in Affiliates									11.0
Gain on sale of assets									2.0
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member] | FMF Littleton [Member]
Schedule of Equity Method Investments [Line Items]
Proceeds from Divestiture of Businesses and Interests in Affiliates									19.2
Gain on sale of assets									14.6
Real estate and land option charges									$ 3.0