Fair Value Measurements - Narrative (Details) - Investments with Difference Between Model Price and Third-Party Price	Jun. 30, 2018 USD ($) loan	Dec. 31, 2017 USD ($) loan
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	17	4
Excess (deficit) of third party prices over internally developed price	$ (18,000,000)	$ (3,000,000)
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	39,000,000
Internal Assessment | Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	197,000,000	25,000,000
Internal Assessment | Securitized Loans Held For Investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	1,190,000,000	0
Third Party Assessment | Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	215,000,000	$ 28,000,000
Third Party Assessment | Securitized Loans Held For Investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 1,150,000,000