Notes Payable - Schedule of Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Series A Senior Notes [Member]
Debt Instrument [Line Items]
Effective Annual Interest Rate	5.16%
Carrying Value	$ 37,800	$ 37,776
Series B Senior Notes [Member]
Debt Instrument [Line Items]
Effective Annual Interest Rate	5.44%
Carrying Value	$ 66,493	66,466
Series C Senior Notes [Member]
Debt Instrument [Line Items]
Effective Annual Interest Rate	5.64%
Carrying Value	$ 47,555	47,542
Series D Senior Notes [Member]
Debt Instrument [Line Items]
Effective Annual Interest Rate	5.72%
Carrying Value	$ 16,831	16,828
Senior Notes [Member]
Debt Instrument [Line Items]
Carrying Value	$ 168,679	$ 168,612