Senior Notes, Secured, and Unsecured Indebtedness - Schedule of Maturities of Construction Notes Payable (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Total notes payable	$ 161,011	$ 155,768
Construction Loans
Debt Instrument [Line Items]
Total notes payable	220,100
Outstanding	105,800
Construction Loans | March 2016 Construction Notes Payable
Debt Instrument [Line Items]
Total notes payable	33,400
Outstanding	$ 16,700
Current Rate	4.24%
Construction Loans | January 2016 Construction Notes Payable
Debt Instrument [Line Items]
Total notes payable	$ 35,000
Outstanding	$ 31,800
Current Rate	4.48%
Construction Loans | November 2015 Construction Notes Payable
Debt Instrument [Line Items]
Total notes payable	$ 42,500
Outstanding	$ 19,300
Current Rate	5.25%
Construction Loans | July 2015 Construction Notes Payable
Debt Instrument [Line Items]
Total notes payable	$ 15,000
Outstanding	$ 3,600
Current Rate	4.75%
Construction Loans | November 2014 Construction Notes Payable
Debt Instrument [Line Items]
Total notes payable	$ 7,000
Outstanding	$ 3,700
Current Rate	4.75%
Construction Loans | November 2014 Construction Notes Payable
Debt Instrument [Line Items]
Total notes payable	$ 15,000
Outstanding	$ 0
Current Rate	4.75%
Construction Loans | March 2014 Construction Notes Payable
Debt Instrument [Line Items]
Total notes payable	$ 26,000
Outstanding	$ 2,400
Current Rate	4.24%
Construction Loans | July 2017 Construction Notes Payable
Debt Instrument [Line Items]
Total notes payable	$ 46,200
Outstanding	$ 28,300
Current Rate	4.46%