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Corporate Borrowings - Narrative (Details) - USD ($)
12 Months Ended
Apr. 30, 2019
Dec. 31, 2017
Sep. 30, 2019
Dec. 31, 2018
Credit Agreement - Revolver        
Debt Instrument [Line Items]        
Maximum borrowing capacity     $ 150,000,000  
Contingent increase in borrowing capacity     25,000,000  
Outstanding borrowings under credit facility     500,000 $ 11,000,000
Credit facility, remaining borrowing capacity     149,500,000  
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