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Concentrations of Credit Risk (Details) - Fannie Mae DUS and Freddie Mac TAH Loans - Liabilities - Credit Concentration Risk - USD ($)
$ in Billions
3 Months Ended 12 Months Ended
Mar. 31, 2021
Dec. 31, 2020
Concentration Risk    
Maximum DB Cayman credit protection $ 7.4 $ 7.2
California    
Concentration Risk    
Concentration risk percentage (in percentage) 21.00% 21.00%
Texas    
Concentration Risk    
Concentration risk percentage (in percentage) 14.00% 14.00%