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Transfers of financial assets and servicing assets - Additional Information (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2022
Sep. 30, 2021
Sep. 30, 2022
Sep. 30, 2021
Dec. 31, 2021
Securitization Financial Asset For Which Transfer Is Accounted As Sale Line Items          
Weighted average mortgage servicing fees 0.31% 0.30% 0.31% 0.30%  
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 31,805,921,000   $ 31,805,921,000   $ 29,506,225,000
Buy Back Option Program          
Securitization Financial Asset For Which Transfer Is Accounted As Sale Line Items          
Mortgage loans related to buy-back option program 9,000,000   9,000,000   13,000,000
Loans with recourse          
Securitization Financial Asset For Which Transfer Is Accounted As Sale Line Items          
Loans serviced 700,000,000   700,000,000   700,000,000
GNMA          
Securitization Financial Asset For Which Transfer Is Accounted As Sale Line Items          
Repurchased loans     49,000,000 80,000,000  
Residential Mortgage          
Securitization Financial Asset For Which Transfer Is Accounted As Sale Line Items          
Securitization Financial Asset For Which Transfer Is Accounted As Sale Gain Loss On Sale 1,000,000.0 5,300,000 800,000 13,700,000  
Mortgage loans related to buy-back option program 1,500,000,000   1,500,000,000   1,600,000,000
Serviced mortgage loans          
Securitization Financial Asset For Which Transfer Is Accounted As Sale Line Items          
Servicing rights on whole loan sales, principal balance 50,000,000 114,000,000 50,000,000 114,000,000  
Securitization Financial Asset For Which Transfer Is Accounted As Sale Gain Loss On Sale     800,000 2,600,000  
Loans serviced 11,400,000,000   11,400,000,000   $ 12,100,000,000
Serviced mortgage loans | Loans with recourse          
Securitization Financial Asset For Which Transfer Is Accounted As Sale Line Items          
Repurchased loans $ 1,000,000 $ 2,000,000 $ 6,000,000 $ 17,000,000