Mortgage Servicing Rights ("MSRs") and Related Liabilities - Fair Value Sensitivity Analysis (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
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	$ (147)	$ (129)
Total Prepayment Speeds, 20% Adverse Change	(283)	(250)
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	50	38
Total Prepayment Speeds, 20% Adverse Change	106	81
100 Basis Points | 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]
Discount Rate, Adverse Change	(125)	(137)
100 Basis Points | 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]
Discount Rate, Adverse Change	50	47
200 Basis Points | 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]
Discount Rate, Adverse Change	(241)	(265)
200 Basis Points | 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]
Discount Rate, Adverse Change	$ 104	$ 99