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Fair Value of Financial Instruments - Additional Information (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)
Residential loans
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value option elected aggregate carrying amount, asset	$ 1,790	$ 5,520
Business purpose loans, at fair value
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value option elected aggregate carrying amount, asset	126
Multifamily loans, held-for-investment, at fair value
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value option elected aggregate carrying amount, asset	$ 963
Real estate securities | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Dealer marks of securities (as a percent)	79.00%	79.00%
Carrying value for which dealer quotes have been received (as a percent)	86.00%	86.00%
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	$ 32	$ 105
Residential Subordinate Securities | Subordinate Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value option elected aggregate carrying amount, asset	$ 128	$ 417
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%

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