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Debt Instruments - Cash Collateralized Revolving Credit Agreement (Details) - Credit Agreement - USD ($)
12 Months Ended
Dec. 31, 2019
Nov. 30, 2019
Dec. 31, 2018
Line of Credit Facility [Line Items]      
Amount of debt outstanding $ 20,300,000   $ 0
Line of Credit      
Line of Credit Facility [Line Items]      
Debt facility, maximum borrowing amount   $ 70,000,000.0  
Commitment fee percentage 0.20%    
Amount of debt outstanding $ 20,300,000    
Line of Credit | LIBOR      
Line of Credit Facility [Line Items]      
Basis spread on variable rate 1.50%