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Fair Value of Financial Instruments - Additional Information (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Residential loans |  |
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |  |
Fair value option elected aggregate carrying amount, asset | $ 5,640 |
Real estate securities | Maximum |  |
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |  |
Dealer marks of securities | 80.00% |
Percentage of carrying value for which dealer quotes were received on securities | 85.00% |
Percentage difference of internal valuation than dealer marks | 1.00% |
Residential Senior Securities |  |
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |  |
Fair value option elected aggregate carrying amount, asset | $ 47 | $ 5
Residential Subordinate Securities | Subordinate Securities |  |
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |  |
Fair value option elected aggregate carrying amount, asset | 594 | 288
MSRs |  |
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |  |
Fair value option elected aggregate carrying amount, asset | $ 8 | 25
MSRs | Maximum |  |
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |  |
Percentage difference of internal valuation than dealer marks | 2.00% |
Residential Loans |  |
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |  |
Principal value | $ 5,640 | $ 4,850

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