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FINANCING ACTIVITIES AND OFF-BALANCE SHEET RISK - NARRATIVE (Details) - USD ($)
1 Months Ended 6 Months Ended
Oct. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Jun. 30, 2021
Apr. 30, 2021
Dec. 31, 2020
Oct. 31, 2020
Jun. 30, 2020
Dec. 31, 2019
Debt Instrument [Line Items]                  
Settlement date basis, equities       2 days          
Settlement date basis, options       1 day          
Revolving Credit Facility | October 2019 Credit Facility | Line of Credit                  
Debt Instrument [Line Items]                  
Line of credit facility, amount entered into $ 200,000,000                
Commitment fee percentage 0.10%                
Outstanding borrowings, long-term       $ 0   $ 0     $ 0
Revolving Credit Facility | October 2019 Credit Facility | Eurodollar | Line of Credit                  
Debt Instrument [Line Items]                  
Variable rate on loan 1.00%                
Revolving Credit Facility | October 2019 Credit Facility | Federal Reserve Bank of New York Rate | Line of Credit                  
Debt Instrument [Line Items]                  
Variable rate on loan 0.50%                
Revolving Credit Facility | October 2019 Credit Facility | Eurodollar, One Month Interest Period | Line of Credit                  
Debt Instrument [Line Items]                  
Variable rate on loan 1.00%                
Revolving Credit Facility | October 2019 Credit Facility, As Amended | Line of Credit                  
Debt Instrument [Line Items]                  
Line of credit facility, amount entered into         $ 625,000,000   $ 600,000,000    
Revolving Credit Facility | Line of Credit | June 2019 Credit Facility                  
Debt Instrument [Line Items]                  
Line of credit facility, amount entered into     $ 250,000,000            
Commitment fee percentage     0.35%            
Revolving Credit Facility | Line of Credit | June 2019 Credit Facility | Federal Funds Rate                  
Debt Instrument [Line Items]                  
Variable rate on loan     1.25%            
Revolving Credit Facility | Line of Credit | September 2019 Credit Facility                  
Debt Instrument [Line Items]                  
Line of credit facility, amount entered into   $ 400,000,000              
Outstanding borrowings, short-term           $ 0     $ 0
Commitment fee percentage   0.35%              
Revolving Credit Facility | Line of Credit | September 2019 Credit Facility | Federal Funds Rate                  
Debt Instrument [Line Items]                  
Variable rate on loan   1.25%              
Revolving Credit Facility | Line of Credit | September 2019 Credit Facility, As Amended                  
Debt Instrument [Line Items]                  
Line of credit facility, amount entered into               $ 550,000,000