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Transfers of financial assets and servicing assets - Additional Information (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Dec. 31, 2022
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]          
Weighted average mortgage servicing fees 0.31% 0.31% 0.31% 0.31%  
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Liabilities Incurred $ 0 $ 0 $ 0 $ 0  
Sensitivity Analysis Of Fair Value Of Interests Continued To Be Held By Transfer or Servicing Assets Or Liabilities Description Of Objectives Methodology And Limitations The sensitivity analyses presented in the table above for servicing rights are hypothetical and should be used with caution. As the figures indicate, changes in fair value based on a 10 and 20 percent variation in assumptions generally cannot be extrapolated because the relationship of the change in assumption to the change in fair value may not be linear. Also, in the sensitivity tables included herein, the effect of a variation in a particular assumption on the fair value of the retained interest is calculated without changing any other assumption. In reality, changes in one factor may result in changes in another (for example, increases in market interest rates may result in lower prepayments and increased credit losses), which might magnify or counteract the sensitivities.        
Mortgage loans related to buy-back option program $ 34,369,775,000   34,369,775,000   $ 32,372,925,000
Buy Back Option Program          
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]          
Mortgage loans related to buy-back option program 8,000,000   8,000,000   14,000,000
Loans with recourse          
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]          
Loans serviced 600,000,000   600,000,000   600,000,000
GNMA          
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]          
Repurchased loans     34,000,000 49,000,000  
Residential Mortgage          
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]          
Mortgage loans related to buy-back option program 1,700,000,000   1,700,000,000   1,600,000,000
Serviced mortgage loans          
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]          
Servicing rights on whole loan sales, principal balance 39,000,000 50,000,000 39,000,000 50,000,000  
Mortgage securitizations, net gains     600,000 800,000  
Loans serviced 10,100,000,000   10,100,000,000   $ 11,100,000,000
Serviced mortgage loans | Loans with recourse          
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]          
Repurchased loans 400,000 $ 1,000,000 2,000,000 $ 6,000,000  
Serviced mortgage loans | Servicing contract termination          
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]          
Servicing rights on whole loan sales, principal balance 260,000,000   260,000,000    
mortgage servicing right fair value $ 2,000,000   $ 2,000,000