Senior Notes, Secured, and Unsecured Indebtedness - Schedule of Maturities of Construction Notes Payable (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total notes payable	$ 172,246	$ 94,515
Construction Loans
Debt Instrument [Line Items]
Total notes payable	143,900
Outstanding	85,000
Construction Loans | March 2016 Construction Notes Payable
Debt Instrument [Line Items]
Total notes payable	33,400
Outstanding	$ 1,200
Current Rate	4.76%
Construction Loans | January 2016 Construction Notes Payable
Debt Instrument [Line Items]
Total notes payable	$ 35,000
Outstanding	$ 29,000
Current Rate	5.13%
Construction Loans | November 2015 Construction Notes Payable
Debt Instrument [Line Items]
Total notes payable	$ 42,500
Outstanding	$ 16,000
Current Rate	5.75%
Construction Loans | November 2014 Construction Notes Payable
Debt Instrument [Line Items]
Total notes payable	$ 7,000
Outstanding	$ 1,400
Current Rate	5.25%
Construction Loans | March 2014 Construction Notes Payable
Debt Instrument [Line Items]
Total notes payable	$ 26,000
Outstanding	$ 500
Current Rate	4.87%
Construction Loans | July 2017 Construction Notes Payable
Debt Instrument [Line Items]
Total notes payable	$ 66,200
Outstanding	$ 36,900
Current Rate	4.81%