Concentrations of Credit Risk (Details) - Fannie Mae DUS and Freddie Mac TAH Loans - Liabilities - Credit Concentration Risk - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2020	Dec. 31, 2019
Concentration Risk
Maximum pre-credit enhancement loss exposure	$ 6,700,000,000	$ 5,800,000,000
California
Concentration Risk
Concentration risk percentage (in percentage)	22.00%	21.00%
Texas
Concentration Risk
Concentration risk percentage (in percentage)	14.00%	16.00%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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