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Real Estate Securities - Additional Information (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) investment	Dec. 31, 2018 USD ($) investment
Investment Holdings [Line Items]
Trading securities	$ 860,540,000	$ 1,118,612,000
Trading securities acquired	367,000,000	688,000,000
Trading securities sold	593,000,000	415,000,000
Change in trading securities	56,000,000	(8,000,000)
Available-for-sale securities	239,334,000	333,882,000
AFS securities acquired	27,000,000	8,000,000
AFS securities sold	$ 110,000,000	144,000,000
AFS net realized gains		$ 27,000,000
Number of AFS securities | investment	107	128
Number of securities in unrealized loss position | investment	1	7
Number of securities in a continuous unrealized loss position for twelve consecutive months or longer | investment	1	3
Reclassification out of Accumulated Other Comprehensive Income | Net Unrealized Gains on Available-for-Sale Securities
Investment Holdings [Line Items]
Other than temporary impairment losses, income statement	$ 100,000
Other than temporary impairments	0
Interest Only Senior Trading Securities
Investment Holdings [Line Items]
Debt securities, trading	64,000,000	$ 82,000,000
Unpaid principal balance	84,000,000	78,000,000
Certificated Servicing Strips
Investment Holdings [Line Items]
Debt securities, trading	36,000,000	43,000,000
Third Party Commercial/Multifamily Securities
Investment Holdings [Line Items]
Available-for-sale securities	9,000,000
Re-preforming Loans Securities
Investment Holdings [Line Items]
Debt securities, trading	55,000,000	48,000,000
Residential
Investment Holdings [Line Items]
Available-for-sale securities	230,000,000
Marketable securities, less than five years	0
Marketable securities, due from five to ten years	8,000,000
Subordinate Securities
Investment Holdings [Line Items]
Available-for-sale securities	199,855,000	209,860,000
Mezzanine
Investment Holdings [Line Items]
Available-for-sale securities	13,687,000	36,407,000
Mezzanine | Trading securities
Investment Holdings [Line Items]
Unpaid principal balance	537,000,000	646,000,000
Mezzanine | Sequoia Securities
Investment Holdings [Line Items]
Trading securities	39,000,000	68,000,000
Mezzanine | Subordinate Securities
Investment Holdings [Line Items]
Trading securities	538,489,000	610,819,000
Available-for-sale securities	13,687,000	36,407,000
Mezzanine | Other Third Party Securities
Investment Holdings [Line Items]
Trading securities	104,000,000	114,000,000
Mezzanine | Third Party Commercial/Multifamily Securities
Investment Holdings [Line Items]
Trading securities	395,000,000	429,000,000
Mezzanine | Re-preforming Loans Securities
Investment Holdings [Line Items]
Trading securities	30,000,000	11,000,000
Subordinate | Trading securities
Investment Holdings [Line Items]
Unpaid principal balance	302,000,000	476,000,000
Subordinate | Subordinate Securities
Investment Holdings [Line Items]
Trading securities	171,984,000	349,123,000
Available-for-sale securities	199,855,000	209,860,000
Subordinate | Other Third Party Securities
Investment Holdings [Line Items]
Trading securities	82,000,000	72,000,000
Subordinate | CRT Securities
Investment Holdings [Line Items]
Trading securities	90,000,000	277,000,000
Subordinate | Re-preforming Loans Securities
Investment Holdings [Line Items]
Trading securities	$ 76,000,000	$ 63,000,000

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