Stockholders' Equity (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended							6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2021	Apr. 30, 2021	Sep. 30, 2019	Sep. 30, 2018
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount										$ 1,000.0
Treasury Stock, Shares, Acquired								5.5
Payments for Repurchase of Common Stock								$ 420.2
Stock Repurchase Program, Remaining Authorized Repurchase Amount		$ 115.1						115.1
Divedends declared (in dollars per sh		$ 0.20	$ 0.20	$ 0.175	$ 0.175	$ 0.175	$ 0.175
Cash dividends paid per common share (in dollars per share)		$ 0.20	$ 0.20	$ 0.175	$ 0.175	$ 0.175	$ 0.175
Subsequent Event [Member]
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount									$ 1,000.0
Divedends declared (in dollars per sh	$ 0.20
Forestar Group [Member]
Class of Stock [Line Items]
Equity Securities Registered, Value											$ 500.0
At-the-market Equity Offering Program, Common Stock Available for Issuance		$ 76.3		$ 100.0				76.3
At-the-market Equity Offering Program, Common Stock Issued		1.0
Stock Issued During Period, Value, New Issues		$ 23.3
Common Stock Available for Issuance, Value Remaining		$ 370.6						$ 370.6