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Transfers of financial assets and servicing assets - Additional Information (Details) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
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 $ 63,000,000 $ 57,000,000
Securitization Financial Asset For Which Transfer Is Accounted As Sale Gain Loss On Sale $ 1,600,000 $ 800,000
Weighted average mortgage servicing fees 30.00% 30.00%
Mortgage loans related to buy-back option program $ 27,587,856,000 $ 26,663,713,000
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Liabilities Incurred $ 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 tables 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 1,300,000,000
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 103,000,000 134,000,000
Repurchased Loans 104,000,000 321,000,000
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 17,200,000 8,900,000
Serviced Mortgage Loans    
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]    
Loans serviced 14,800,000,000 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 $ 57,000,000 $ 27,000,000