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Corporate Borrowings - Narrative (Details) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2017
Mar. 31, 2021
Dec. 31, 2020
Feb. 14, 2020
Debt Instrument [Line Items]          
Long-term debt, gross     $ 40,000,000 $ 40,000,000  
Line of Credit | Revolving Credit Facility          
Debt Instrument [Line Items]          
Remaining borrowing capacity     300,000,000    
Credit Agreement - Revolver, 1.12% (variable rate) due 2025          
Debt Instrument [Line Items]          
Outstanding borrowings under credit facility         $ 300,000,000
Credit Agreement | Line of Credit | Revolving Credit Facility          
Debt Instrument [Line Items]          
Contingent increase in borrowing capacity         $ 100,000,000
Credit Agreement - Revolver, 1.12% (variable rate) due 2025 | Line of Credit | Revolving Credit Facility          
Debt Instrument [Line Items]          
Long-term debt, gross     0 0  
Note Purchase Agreement | Secured Debt          
Debt Instrument [Line Items]          
Term loan face value     20,000,000    
Note Purchase Agreement | Secured Debt | Revolving Credit Facility          
Debt Instrument [Line Items]          
Term loan face value     105,000,000    
Prudential Note Purchase Agreement, 3.53% (fixed rate) due 2027 | Secured Debt          
Debt Instrument [Line Items]          
Long-term debt, gross     20,000,000 20,000,000  
Proceeds from issuance of debt   $ 20,000,000      
Prudential Note Purchase Agreement, 3.80% (fixed rate) due 2029 | Secured Debt          
Debt Instrument [Line Items]          
Long-term debt, gross     $ 20,000,000 $ 20,000,000  
Proceeds from issuance of debt $ 20,000,000