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Concentrations of Credit Risk - Other Concentrations (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Concentration Risk [Line Items]    
Estimated benefit included in combined loss reserves $ 559 $ 1,400
Receivable from claims on insured, defaulted loans excluding government insured loans 533 560
Allowance for mortgage insurance receivable $ 479 497
PMI    
Concentration Risk [Line Items]    
Mortgage insurance coverage risk in force percentage to be paid in cash 77.50%  
Triad    
Concentration Risk [Line Items]    
Mortgage insurance coverage risk in force percentage to be paid in cash 75.00%  
Single-family | Guaranty Book of Business    
Concentration Risk [Line Items]    
Primary mortgage insurance coverage risk in force $ 176,587 170,890
Pool mortgage insurance coverage risk in force 261 291
Mortgage insurance coverage risk in force $ 176,848 $ 171,181
Mortgage insurance coverage risk in force as a percentage of book of business 5.00% 5.00%
Single-family | Guaranty Book of Business | Credit Concentration Risk | Group of Largest Mortgage Servicers including Affiliates    
Concentration Risk [Line Items]    
Concentration risk, percentage 44.00% 48.00%
Single-family | Guaranty Book of Business | Insurance Service Provider Concentration Risk    
Concentration Risk [Line Items]    
Concentration risk, percentage 100.00% 100.00%
Single-family | Guaranty Book of Business | Insurance Service Provider Concentration Risk | Arch Capital Group Ltd.    
Concentration Risk [Line Items]    
Concentration risk, percentage 19.00% 21.00%
Single-family | Guaranty Book of Business | Insurance Service Provider Concentration Risk | Mortgage Guaranty Insurance Corp.    
Concentration Risk [Line Items]    
Concentration risk, percentage 19.00% 18.00%
Single-family | Guaranty Book of Business | Insurance Service Provider Concentration Risk | Radian Guaranty, Inc.    
Concentration Risk [Line Items]    
Concentration risk, percentage 17.00% 19.00%
Single-family | Guaranty Book of Business | Insurance Service Provider Concentration Risk | Genworth Mortgage Insurance Corp.    
Concentration Risk [Line Items]    
Concentration risk, percentage 17.00% 16.00%
Single-family | Guaranty Book of Business | Insurance Service Provider Concentration Risk | Essent Guaranty, Inc.    
Concentration Risk [Line Items]    
Concentration risk, percentage 16.00% 16.00%
Single-family | Guaranty Book of Business | Insurance Service Provider Concentration Risk | National Mortgage Insurance Corp.    
Concentration Risk [Line Items]    
Concentration risk, percentage 11.00% 9.00%
Single-family | Guaranty Book of Business | Insurance Service Provider Concentration Risk | Others    
Concentration Risk [Line Items]    
Concentration risk, percentage 1.00% 1.00%
Single-family | Guaranty Book of Business | Insurance Service Provider Concentration Risk | Three Mortgage Insureres    
Concentration Risk [Line Items]    
Concentration risk, percentage 1.00%  
Mortgage insurance coverage risk in force for insurers with credit quality deterioration $ 1,500  
Single-family | Depository Servicer | Guaranty Book of Business | Credit Concentration Risk | Wells Fargo Bank, N.A. (together with its affiliates)    
Concentration Risk [Line Items]    
Concentration risk, percentage 10.00% 13.00%
Single-family | Depository Servicer | Guaranty Book of Business | Credit Concentration Risk | Remaining top five servicers    
Concentration Risk [Line Items]    
Concentration risk, percentage 11.00% 11.00%
Single-family | Non-Depository Servicer | Guaranty Book of Business | Credit Concentration Risk | Remaining top five servicers    
Concentration Risk [Line Items]    
Concentration risk, percentage 23.00% 24.00%
Multifamily    
Concentration Risk [Line Items]    
Maximum potential loss recovery from lenders under risk sharing agreements $ 97,600 $ 92,900
Multifamily | Guaranty Book of Business | Credit Concentration Risk | Group of Largest Mortgage Servicers including Affiliates    
Concentration Risk [Line Items]    
Concentration risk, percentage 47.00% 48.00%
Multifamily | Guaranty Book of Business | Credit Concentration Risk | Walker & Dunlop, Inc.    
Concentration Risk [Line Items]    
Concentration risk, percentage 12.00% 12.00%
Multifamily | Guaranty Book of Business | Credit Concentration Risk | Wells Fargo Bank, N.A. (together with its affiliates)    
Concentration Risk [Line Items]    
Concentration risk, percentage 11.00% 12.00%
Multifamily | Guaranty Book of Business | Credit Concentration Risk | Remaining top five servicers    
Concentration Risk [Line Items]    
Concentration risk, percentage 24.00% 24.00%
Multifamily | Accounts Receivable | Credit Concentration Risk | Five Major Lenders    
Concentration Risk [Line Items]    
Concentration risk, percentage 52.00% 51.00%