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Corporate Borrowings - Narrative (Details) - USD ($)
6 Months Ended 12 Months Ended
Jun. 30, 2020
Dec. 31, 2019
Dec. 31, 2017
Feb. 14, 2020
Debt Instrument [Line Items]        
Outstanding borrowings under credit facility $ 23,662,000 $ 1,000    
Previous Credit Agreement - Revolver, 3.19% (variable rate) due 2022        
Debt Instrument [Line Items]        
Maximum borrowing capacity 150,000,000      
Outstanding borrowings under credit facility 0 300,000    
Credit Agreement - Revolver, 1.20% (variable rate) due 2025        
Debt Instrument [Line Items]        
Outstanding borrowings under credit facility 25,000,000 0   $ 300,000
Contingent increase in borrowing capacity 100,000,000      
Payments of debt issuance costs 1,148,000      
Credit facility, remaining borrowing capacity 275,000,000      
Prudential Note Purchase Agreement, 3.53% (fixed rate) due 2027 | Senior Notes        
Debt Instrument [Line Items]        
Term loan face value $ 75,000,000      
Proceeds from issuance of debt     $ 20,000,000  
Prudential Note Purchase Agreement, 3.80% (fixed rate) due 2029 | Senior Notes        
Debt Instrument [Line Items]        
Proceeds from issuance of debt   $ 20,000,000