Notes Payable and Subordinated Borrowings - Schedule of Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Series A Senior Notes [Member]
Debt Instrument [Line Items]
Effective Annual Interest Rate	5.16%
Carrying Value	$ 37,729	$ 37,684
Series B Senior Notes [Member]
Debt Instrument [Line Items]
Effective Annual Interest Rate	5.44%
Carrying Value	$ 66,410	66,356
Series C Senior Notes [Member]
Debt Instrument [Line Items]
Effective Annual Interest Rate	5.64%
Carrying Value	$ 47,517	47,493
Series D Senior Notes [Member]
Debt Instrument [Line Items]
Effective Annual Interest Rate	5.72%
Carrying Value	$ 16,821	16,814
Senior Notes [Member]
Debt Instrument [Line Items]
Carrying Value	$ 168,477	$ 168,347