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Fair Value of Financial Instruments - Additional Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Residential loans
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair value option elected aggregate carrying amount, asset	$ 1,800
Real estate securities | Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Dealer marks of securities (as a percent)	72.00%
Carrying value for which dealer quotes have been received (as a percent)	79.00%
Difference of internal valuation than dealer marks (as a percent)	1.00%
Residential Senior Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair value option elected aggregate carrying amount, asset	$ 12
Residential Subordinate Securities | Subordinate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair value option elected aggregate carrying amount, asset	$ 128
Mortgage servicing rights | Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Difference of internal valuation than dealer marks (as a percent)	2.00%

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

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