Fair Value of Financial Instruments - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	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