Concentrations of Credit Risk (Details) - Fannie Mae DUS and Freddie Mac TAH Loans - Liabilities - Credit Concentration Risk - USD ($) $ in Billions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Concentration Risk
Maximum pre-credit enhancement loss exposure	$ 7.2	$ 5.8
California
Concentration Risk
Concentration risk percentage (in percentage)	21.00%	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.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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