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Corporate Borrowings - Narrative (Details) - USD ($)
3 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Aug. 23, 2017
Credit Agreement - Revolver, 3.87% (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 $ 22,025,000 $ 11,000,000  
Credit facility, remaining borrowing capacity 127,975,000    
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