Summary of Significant Accounting Policies - Business Acquisitions (Details) $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 02, 2016 Lot Home	Apr. 30, 2015 Lot Home
Business Acquisition [Line Items]
Payments to Acquire Businesses, Net of Cash Acquired			$ 82.2	$ 70.9	$ 244.1
Pacific Ridge Homes [Member]
Business Acquisition [Line Items]
Business Acquisition, Date of Acquisition Agreement		Apr. 24, 2015
Payments to Acquire Businesses, Net of Cash Acquired		$ 70.9
Business Acquisition, Number of Homes Acquired | Home							90
Business Acquisition, Number of Finished Lots Acquired | Lot							350
Business Acquisition, Number of Lots Under Option Contracts | Lot							400
Business Acquisition, Sales Order Backlog Acquired (Homes) | Home							42
Goodwill, Acquired During Period		$ 2.2
Wilson Parker Homes [Member]
Business Acquisition [Line Items]
Business Acquisition, Date of Acquisition Agreement	Sep. 02, 2016
Payments to Acquire Businesses, Net of Cash Acquired	$ 91.9
Business Acquisition, Number of Homes Acquired | Home						380
Business Acquisition, Number of Finished Lots Acquired | Lot						490
Business Acquisition, Number of Lots Under Option Contracts | Lot						1,850
Business Acquisition, Sales Order Backlog Acquired (Homes) | Home						308
Goodwill, Acquired During Period	$ 0.0