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Notes Payable - Additional Information (Details)
1 Months Ended 12 Months Ended
Aug. 01, 2019
USD ($)
May 31, 2018
USD ($)
Mar. 31, 2018
USD ($)
Dec. 31, 2019
Aug. 01, 2019
GBP (£)
Mar. 30, 2016
USD ($)
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 Debt [Member]            
Debt Instrument [Line Items]            
Extinguishment of Debt, Amount   $ 99,000 $ 6,700,000      
Parent Company [Member] | Senior Notes [Member]            
Debt Instrument [Line Items]            
Long-term Debt, Gross           $ 170,000,000
Long-term Debt, Weighted Average Life 12 years          
Long-term Debt, Weighted Average Interest Rate, at Point in Time (as a percent) 4.26%       4.26%  
Parent Company [Member] | Senior Notes [Member] | United States of America, Dollars            
Debt Instrument [Line Items]            
Long-term Debt, Gross $ 175,000,000          
Parent Company [Member] | Senior Notes [Member] | United Kingdom, Pounds            
Debt Instrument [Line Items]            
Long-term Debt, Gross | £         £ 25,000,000  
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%
Parent Company [Member] | Series E Senior Notes [Member]            
Debt Instrument [Line Items]            
Long-term Debt, Gross $ 75,000,000          
Debt Instrument, Interest Rate, Stated Percentage 4.34%       4.34%  
Parent Company [Member] | Series F Senior Notes [Member]            
Debt Instrument [Line Items]            
Long-term Debt, Gross $ 60,000,000          
Debt Instrument, Interest Rate, Stated Percentage 4.44%       4.44%  
Parent Company [Member] | Series G Senior Notes [Member]            
Debt Instrument [Line Items]            
Long-term Debt, Gross $ 40,000,000          
Debt Instrument, Interest Rate, Stated Percentage 4.54%       4.54%  
Parent Company [Member] | Series H Senior Notes [Member]            
Debt Instrument [Line Items]            
Long-term Debt, Gross | £         £ 25,000,000  
Debt Instrument, Interest Rate, Stated Percentage 3.33%       3.33%