Fair Value of Financial Instruments - Additional Information (Details) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)
Residential loans
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value option elected aggregate carrying amount, asset	$ 1,930	$ 3,730
Real estate securities \| Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Dealer marks of securities (as a percent)	75.00%	75.00%
Carrying value for which dealer quotes have been received (as a percent)	84.00%	84.00%
Difference of internal valuation than dealer marks (as a percent)	1.00%	1.00%
Residential Senior Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value option elected aggregate carrying amount, asset	$ 60	$ 72
Residential Subordinate Securities \| Subordinate Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value option elected aggregate carrying amount, asset	$ 161	$ 289
Mortgage servicing rights \| Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Difference of internal valuation than dealer marks (as a percent)	2.00%	2.00%

Fair Value, Option, Assets, Aggregate Carrying Amount During Period

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Percentage Difference Of Internal Valuation Than Dealer Marks

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Percentage of Carrying Value for Which Dealer Quotes Were Received on Securities

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Percentage of Third-party Dealer Quotes Received on Securities.

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