Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions			9 Months Ended	12 Months Ended
	Sep. 30, 2019	Oct. 05, 2017	Sep. 30, 2018	Sep. 30, 2019	Dec. 31, 2017
Class of Stock [Line Items]
Equity Securities Registered, Value			$ 500.0
Proceeds from Issuance of Common Stock			$ 0.0	$ 100.7	$ 0.0
Common Stock, Shares Authorized	$ 394.3			$ 394.3
Shares Issued, Price Per Share	$ 17.50			$ 17.50
D.R. Horton Merger Agreement [Member] | D.R. Horton, Inc. [Member]
Class of Stock [Line Items]
Sale of Stock, Percentage of Ownership after Transaction	66.00%	75.00%
Common Stock
Class of Stock [Line Items]
Proceeds from Issuance of Common Stock				$ 6.0
Shares Issued During Period, New Issues				6,037,500