Investment In Unconsolidated Ventures - Text (Detail)	9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2019 USD ($) venture	Dec. 31, 2017 USD ($)
Gain (Loss) on Disposition of Assets	$ 27,800,000	$ 113,400,000	$ 3,000,000	$ 113,400,000
Asset impairments	300,000		$ 1,300,000	47,200,000
Number of Equity Method Investments | venture			4
Payments to Acquire Equity Method Investments	0		$ 0	4,500,000
Proceeds from Equity Method Investment, Distribution, Return of Capital	800,000		100,000	11,400,000
Equity Method Investment, Summarized Financial Information, Assets	27,500,000		15,300,000
Investment in unconsolidated ventures	11,700,000		7,300,000
Equity Method Investment, Summarized Financial Information, Revenue	22,200,000		1,900,000	65,700,000
Equity Method Investment, Summarized Financial Information, Income (Loss) from Continuing Operations	15,100,000		1,300,000	39,200,000
Income (Loss) from Equity Method Investments	4,800,000	$ 10,900,000	500,000	17,800,000
Lm Land Holdings [Member]
Equity Method Investment, Summarized Financial Information, Assets	21,600,000
Equity Method Investment Summarized Financial Information, Equity	21,200,000
Investment in unconsolidated ventures	8,900,000
Equity Method Investment, Summarized Financial Information, Revenue	17,400,000
Equity Method Investment, Summarized Financial Information, Income (Loss) from Continuing Operations	18,100,000
Income (Loss) from Equity Method Investments	6,400,000
FOR/SR Forsyth LLC [Member]
Proceeds from Divestiture of Businesses and Interests in Affiliates	11,000,000
Gain (Loss) on Disposition of Assets	2,000,000
FMF Littleton [Member]
Proceeds from Divestiture of Businesses and Interests in Affiliates	19,200,000
Gain (Loss) on Disposition of Assets	14,600,000
CREA FMF [Member]
Asset impairments	3,000,000
Accounts Payable and Accrued Liabilities [Member]
Equity Method Investment, Summarized Financial Information, Liabilities	600,000		300,000
Accounts Payable and Accrued Liabilities [Member] | Lm Land Holdings [Member]
Equity Method Investment, Summarized Financial Information, Liabilities	400,000
Gain (Loss) on Investments [Member] | Lm Land Holdings [Member]
Equity Method Investment, Summarized Financial Information, Income (Loss) from Continuing Operations	5,700,000
Equity Method Investments [Member]
Proceeds from Equity Method Investment, Distribution, Return of Capital	$ 4,300,000		$ 5,000,000
Equity Method Investments [Member] | Two Hundred Forty Two Llc [Member]
Gain (Loss) on Disposition of Assets				19,000,000
Proceeds from Sale of Real Estate				71,800,000
Proceeds from Equity Method Investment, Distribution, Return of Capital				12,000,000
Income (Loss) from Equity Method Investments				7,800,000
Equity Method Investments [Member] | CL Realty [Member]
Gain (Loss) on Disposition of Assets				6,600,000
Proceeds from Sale of Real Estate				9,700,000
Proceeds from Equity Method Investment, Distribution, Return of Capital				4,300,000
Income (Loss) from Equity Method Investments				$ 3,300,000
Starwood Land, L.P.. [Member] | Equity Method Investments [Member]
Number of Equity Method Investments Sold	8