Segment Information - Items Not Allocated to Segments (Detail) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2019	Dec. 31, 2017
Segment Reporting Information [Line Items]
Gain of sale of assets	$ 27.8	$ 113.4	$ 3.0	$ 113.4
Interest expense	3.7	6.4	0.0	8.5
Loss on extinguishment of debt, net	0.0		0.0	(0.6)
Other corporate non-operating income	(6.4)	$ (2.4)	$ (5.5)	(3.6)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(43.5)			(50.1)
Items Not Allocated To Segments
Segment Reporting Information [Line Items]
Selling, general and administrative expense	11.5			50.4
Stock-based and long-term incentive compensation expense	0.5			7.2
Gain (Loss) on Sale of Properties	0.0			(28.7)
Gain of sale of assets	0.0			28.7
Interest expense	3.7			8.5
Loss on extinguishment of debt, net	0.0			0.6
Other corporate non-operating income	(4.6)			(1.6)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	$ 11.1			36.4
General and Administrative Expense | Starwood Capital Group [Member]
Segment Reporting Information [Line Items]
Loss on Contract Termination				20.0
D.R. Horton Merger Agreement [Member] | General and Administrative Expense
Segment Reporting Information [Line Items]
Payments for Merger Related Costs				37.2
Professional Fees				11.8
Severance Costs				$ 5.4