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Notes Payable and Subordinated Borrowings - Additional Information (Details) - USD ($)
1 Months Ended 6 Months Ended
Mar. 30, 2016
May 31, 2018
Mar. 31, 2018
Apr. 30, 2017
Feb. 28, 2017
Jun. 30, 2018
Dec. 31, 2017
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           $ 0 $ 6,799,000
Subordinated Debt [Member]              
Debt Instrument [Line Items]              
Extinguishment of Debt, Amount   $ 99,000 $ 6,700,000 $ 3,751,000 $ 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%