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Transfers of financial assets and servicing assets - Additional Information (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]          
Weighted average mortgage servicing fees 0.30% 0.30% 0.30% 0.30%  
Mortgage loans related to buy-back option program $ 27,181,241,000   $ 27,181,241,000   $ 26,663,713,000
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.    
Loans with Recourse          
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]          
Loans serviced 1,200,000,000   $ 1,200,000,000   1,300,000,000
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 99,000,000   99,000,000   134,000,000
Repurchased Loans     88,000,000 264,000,000  
Residential Mortgage          
Securitization or Asset-backed Financing Arrangement, 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 4,900,000 2,900,000 13,400,000 6,200,000  
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 45,000,000 43,000,000 45,000,000 43,000,000  
Securitization Financial Asset For Which Transfer Is Accounted As Sale Gain Loss On Sale     1,300,000 600,000  
Loans serviced 15,000,000,000.0   15,000,000,000.0   $ 15,700,000,000
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 $ 15,000,000 $ 4,000,000 $ 38,000,000 $ 13,000,000