Notes Payable - Terms (Details) - Homebuilding [Member] - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Senior Note Twenty Nine [Member]
Summary of unsecured homebuilding notes payable outstanding
Date Issued	Feb. 01, 2014
Date Due	Mar. 01, 2019
Redeemable Prior to Maturity	Yes
Effective Interest Rate	3.90%
Debt Instrument, Face Amount	$ 500.0
Senior Note Member Thirty [Member]
Summary of unsecured homebuilding notes payable outstanding
Date Issued	Feb. 01, 2015
Date Due	Feb. 15, 2020
Redeemable Prior to Maturity	Yes
Effective Interest Rate	4.20%
Debt Instrument, Face Amount	$ 500.0
Senior Note Member Thirty-One [Member]
Summary of unsecured homebuilding notes payable outstanding
Date Issued	Dec. 01, 2017
Date Due	Dec. 01, 2020
Redeemable Prior to Maturity	Yes
Effective Interest Rate	2.80%
Debt Instrument, Face Amount	$ 400.0	$ 400.0
Senior Note Member Twenty Five [Member]
Summary of unsecured homebuilding notes payable outstanding
Date Issued	Sep. 01, 2012
Date Due	Sep. 15, 2022
Redeemable Prior to Maturity	Yes
Effective Interest Rate	4.50%
Debt Instrument, Face Amount	$ 350.0
Senior Note Twenty Seven [Member]
Summary of unsecured homebuilding notes payable outstanding
Date Issued	Feb. 01, 2013
Date Due	Feb. 15, 2023
Redeemable Prior to Maturity	Yes
Effective Interest Rate	4.90%
Debt Instrument, Face Amount	$ 300.0
SeniorNoteTwentyEight [Member]
Summary of unsecured homebuilding notes payable outstanding
Date Issued	Aug. 01, 2013
Date Due	Aug. 15, 2023
Redeemable Prior to Maturity	Yes
Effective Interest Rate	5.90%
Debt Instrument, Face Amount	$ 400.0
Senior Notes Redeemable Prior to Maturity [Member]
Summary of unsecured homebuilding notes payable outstanding
Debt Instrument, Redemption Price, Percentage	100.00%