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Notes Payable and Subordinated Borrowings - Additional Information (Details) - USD ($)
1 Months Ended 12 Months Ended
Mar. 30, 2016
Apr. 30, 2017
Feb. 28, 2017
Apr. 30, 2016
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]            
Minimum Repayment of Aggregate Principal Amount of Senior Notes (as a percent)         5.00%  
Outstanding Principal Amount of Senior Notes (as a percent)         100.00%  
Subordinated Borrowing, Interest Rate (as a percent)         5.50%  
Subordinated Borrowings         $ 6,799,000 $ 16,550,000
Subordinated Debt [Member]            
Debt Instrument [Line Items]            
Extinguishment of Debt, Amount   $ 3,751,000 $ 6,000,000 $ 6,000,000    
Parent Company [Member] | Senior Notes [Member]            
Debt Instrument [Line Items]            
Extinguishment of Debt, Amount $ 120,000,000          
Parent Company [Member] | Senior Notes [Member]            
Debt Instrument [Line Items]            
Long-term Debt, Gross 170,000,000          
Parent Company [Member] | Series A Senior Notes [Member]            
Debt Instrument [Line Items]            
Long-term Debt, Gross $ 38,000,000          
Debt Instrument, Interest Rate, Stated Percentage 4.88%          
Parent Company [Member] | Series B Senior Notes [Member]            
Debt Instrument [Line Items]            
Long-term Debt, Gross $ 67,000,000          
Debt Instrument, Interest Rate, Stated Percentage 5.23%          
Parent Company [Member] | Series C Senior Notes [Member]            
Debt Instrument [Line Items]            
Long-term Debt, Gross $ 48,000,000          
Debt Instrument, Interest Rate, Stated Percentage 5.48%          
Parent Company [Member] | Series D Senior Notes [Member]            
Debt Instrument [Line Items]            
Long-term Debt, Gross $ 17,000,000          
Debt Instrument, Interest Rate, Stated Percentage 5.58%