Segment Information - Items Not Allocated to Segments (Detail) - USD ($)		9 Months Ended		12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
General and administrative expense		$ (11,811,000)	$ (38,403,000)	$ (56,531,000)	$ (21,597,000)
Gain (Loss) on Sale of Properties		27,756,000		113,411,000	166,747,000
Gain of sale of assets		27,756,000	113,411,000	113,411,000	166,747,000
Interest expense		(3,741,000)	(6,439,000)	(8,532,000)	(19,985,000)
Loss on extinguishment of debt, net		0		(611,000)	(35,864,000)
Other corporate non-operating income		6,395,000	$ 2,438,000	3,600,000	1,718,000
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest		43,442,000		50,043,000	90,815,000
Items Not Allocated To Segments
Segment Reporting Information [Line Items]
General and administrative expense		(11,443,000)		(50,354,000)	(18,274,000)
Share-based and long-term incentive compensation expense		(512,000)		(7,201,000)	(4,425,000)
Gain (Loss) on Sale of Properties		0		28,674,000	48,891,000
Interest expense		(3,741,000)		(8,532,000)	(19,985,000)
Loss on extinguishment of debt, net		0		(611,000)	(35,864,000)
Other corporate non-operating income		4,624,000		1,627,000	350,000
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	[1]	$ (11,072,000)		(36,397,000)	$ (29,307,000)
General and Administrative Expense | Starwood Capital Group [Member]
Segment Reporting Information [Line Items]
Loss on Contract Termination				20,000,000
D.R. Horton Merger Agreement [Member] | General and Administrative Expense
Segment Reporting Information [Line Items]
Payments for Merger Related Costs				37,216,000
Share-based and long-term incentive compensation expense				(4,349,000)
Professional Fees				11,787,000
Severance Costs				$ 5,429,000

[1] Items not allocated to segments consist of: For the Nine Months Ended September 30, 2018 For the Twelve Months Ended December 31, 2017 2016 (In thousands)General and administrative expense$(11,443) $(50,354) $(18,274)Share-based and long-term incentive compensation expense(512) (7,201) (4,425)Gain on sale of assets— 28,674 48,891Interest expense(3,741) (8,532) (19,985)Loss on extinguishment of debt, net— (611) (35,864)Interest and other income4,624 1,627 350 $(11,072) $(36,397) $(29,307)