Mortgage Servicing Rights and Related Liabilities - Fair Value Sensitivity Analysis (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Excess Spread Financing
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Total prepayment speeds, 10% adverse change	$ 46	$ 38
Total prepayment speeds, 20% adverse change	96	81
Excess Spread Financing | 100 bps Adverse Change
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Discount rate, adverse change	46	47
Excess Spread Financing | 200 bps Adverse Change
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Discount rate, adverse change	95	99
Forward mortgage servicing rights
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Total prepayment speeds, 10% adverse change	(165)	(129)
Total prepayment speeds, 20% adverse change	(317)	(250)
Forward mortgage servicing rights | 100 bps Adverse Change
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Discount rate, adverse change	(127)	(137)
Forward mortgage servicing rights | 200 bps Adverse Change
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Discount rate, adverse change	$ (245)	$ (265)