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Corporate Borrowings - Narrative (Details) - USD ($)
6 Months Ended
Apr. 30, 2019
Jun. 30, 2019
Dec. 31, 2018
Credit Agreement - Revolver, 3.80% (variable rate) due 2022      
Debt Instrument [Line Items]      
Maximum borrowing capacity   $ 150,000,000  
Contingent increase in borrowing capacity   25,000,000  
Outstanding borrowings under credit facility   1,600,000 $ 11,000,000
Credit facility, remaining borrowing capacity   148,400,000  
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