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Corporate Borrowings - Narrative (Details) - USD ($)
Aug. 23, 2017
Sep. 30, 2018
Dec. 31, 2017
Credit Agreement - Revolver, 3.593% (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   $ 24,044,000 $ 3,298,000
Credit facility, remaining borrowing capacity   $ 125,956,000  
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