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Mortgage Servicing Rights - Summary of Adverse Changes to Weighted-Average Assumptions on the Fair Value of Servicing Rights (Details) - Mortgage servicing rights - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Actual    
Option adjusted spread (in percentage) 7.98% 5.34%
Constant prepayment rate (in percentage) 10.53% 10.59%
Weighted average cost to service per loan (in USD per share) $ 81.24 $ 84.41
Fair value impact due to 10% adverse change    
Option adjusted spread $ 321 $ 284
Constant prepayment rate 305 271
Weighted average cost to service per loan (in dollars per share) 325 285
Fair value impact due to 20% adverse change    
Option adjusted spread 313 280
Constant prepayment rate 283 257
Weighted average cost to service per loan (in dollars per share) $ 321 $ 282