Notes Payable and Subordinated Borrowings - Schedule of Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Series A Senior Notes [Member]
Debt Instrument [Line Items]
Effective Annual Interest Rate	5.16%
Carrying Value	$ 37,684	$ 37,597
Series B Senior Notes [Member]
Debt Instrument [Line Items]
Effective Annual Interest Rate	5.44%
Carrying Value	$ 66,356	66,254
Series C Senior Notes [Member]
Debt Instrument [Line Items]
Effective Annual Interest Rate	5.64%
Carrying Value	$ 47,493	47,445
Series D Senior Notes [Member]
Debt Instrument [Line Items]
Effective Annual Interest Rate	5.72%
Carrying Value	$ 16,814	16,801
Senior Notes [Member]
Debt Instrument [Line Items]
Carrying Value	$ 168,347	$ 168,097