Investment In Unconsolidated Ventures - Text (Detail)		3 Months Ended							9 Months Ended		12 Months Ended
	Feb. 08, 2018 venture	Sep. 30, 2018 USD ($) venture	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) a	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2018 USD ($) venture	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) a	Dec. 31, 2016 USD ($)
Schedule of Equity Method Investments [Line Items]
Number of Equity Method Investments | venture		5							5
Venture Assets		$ 27,537,000			$ 203,754,000				$ 27,537,000		$ 203,754,000
Investment in unconsolidated ventures		11,721,000			64,579,000				11,721,000		64,579,000	$ 77,611,000
Revenues									22,215,000		65,748,000	46,130,000
Earnings (Loss)									15,112,000		39,150,000	9,119,000
Equity in earnings of unconsolidated ventures		2,266,000	$ 1,000,000	$ 1,529,000	$ 7,026,000	$ 1,764,000	$ 2,747,000	$ 6,362,000	4,795,000	$ 10,873,000	17,899,000	6,123,000
Payments to Acquire Equity Method Investments									0		4,548,000	6,089,000
Distributions Of Return On Investments And Earnings									4,292,000		34,439,000	13,419,000
Gain of sale of assets									27,756,000	$ 113,411,000	113,411,000	166,747,000
Asset impairments									338,000		47,172,000	60,939,000
Construction Revenue									0		741,000	2,466,000
Lm Land Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Venture Assets		21,618,000							21,618,000
Venture Equity		21,209,000							21,209,000
Investment in unconsolidated ventures		$ 8,912,000							8,912,000
Revenues									17,425,000
Earnings (Loss)									18,135,000
Equity in earnings of unconsolidated ventures									6,352,000
Gain (Loss) on Investments [Member] | Lm Land Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Earnings (Loss)									5,679,000
Starwood Land, L.P.. [Member]
Schedule of Equity Method Investments [Line Items]
Number of Equity Method Investments Sold | venture	8
Equity Method Investments [Member]
Schedule of Equity Method Investments [Line Items]
Asset impairments									3,045,000
Equity Method Investments [Member] | FOR/SR Forsyth LLC [Member]
Schedule of Equity Method Investments [Line Items]
Proceeds from Divestiture of Businesses and Interests in Affiliates									11,049,000
Gain of sale of assets									2,030,000
Equity Method Investments [Member] | FMF Littleton [Member]
Schedule of Equity Method Investments [Line Items]
Proceeds from Divestiture of Businesses and Interests in Affiliates									19,226,000
Gain of sale of assets									$ 14,573,000
Equity Method Investments [Member] | CREA FMF [Member]
Schedule of Equity Method Investments [Line Items]
Equity in earnings of unconsolidated ventures											7,783,000
Distributions Of Return On Investments And Earnings											11,956,000
Gain of sale of assets											18,986,000
Proceeds from Sale of Real Estate											71,750,000
Equity Method Investments [Member] | Two Hundred Forty Two Llc [Member]
Schedule of Equity Method Investments [Line Items]
Equity in earnings of unconsolidated ventures											3,306,000
Distributions Of Return On Investments And Earnings											$ 4,348,000
Area of Land | a					46						46
Gain of sale of assets											$ 6,612,000
Proceeds from Sale of Real Estate											9,719,000
Equity Method Investments [Member] | CL Realty [Member]
Schedule of Equity Method Investments [Line Items]
Distributions Of Return On Investments And Earnings											1,200,000
Asset impairments											3,756,000
Proceeds from Sale of Real Estate											2,400,000
Equity Method Investments [Member] | Fmf Peakview Llc [Member]
Schedule of Equity Method Investments [Line Items]
Gain of sale of assets												10,363,000
Proceeds from Sale of Real Estate												$ 13,917,000
Variable Interest Entity, Primary Beneficiary [Member]
Schedule of Equity Method Investments [Line Items]
Variable Interest Entity, Qualitative or Quantitative Information, Number | venture		0							0
Accounts Payable and Accrued Liabilities [Member]
Schedule of Equity Method Investments [Line Items]
Venture Liabilities		$ 651,000			$ 13,101,000				$ 651,000		$ 13,101,000
Accounts Payable and Accrued Liabilities [Member] | Lm Land Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Venture Liabilities		$ 409,000							$ 409,000