Fair Value Measurements - Narrative (Details) $ in Millions	Dec. 31, 2019 USD ($) loan	Dec. 31, 2018 USD ($) loan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of loans | loan	139,194	140,345
Investments with Difference Between Model Price and Third-Party Price | Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans internally valued lower than third party prices | loan	6	19
Excess (deficit) of third party prices over internally developed price	$ 3	$ 4
Investments with Difference Between Model Price and Third-Party Price | Securitized Loans Held For Investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Excess (deficit) of third party prices over internally developed price	20	3
Internal Assessment | Investments with Difference Between Model Price and Third-Party Price | Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	22	127
Internal Assessment | Investments with Difference Between Model Price and Third-Party Price | Securitized Loans Held For Investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 147	$ 489
Number of loans | loan	1	3
Third Party Assessment | Investments with Difference Between Model Price and Third-Party Price | Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 19	$ 123
Third Party Assessment | Investments with Difference Between Model Price and Third-Party Price | Securitized Loans Held For Investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Excess (deficit) of third party prices over internally developed price	$ 127	$ 486