Residential Loans - Characteristics of Loans Held-for-Investment (Details) - Held-for-investment at fair value $ in Thousands	3 Months Ended	12 Months Ended
Sep. 30, 2021 USD ($) loan	Dec. 31, 2020 USD ($) loan
Legacy Sequoia
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of loans \| loan	1,653	1,908
Unpaid principal balance	$ 278,815	$ 333,474
Fair value of loans	$ 242,234	$ 285,935
Number of loans in foreclosure \| loan	18	21
Unpaid principal balance of loans in foreclosure	$ 4,416	$ 4,939
Sequoia
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of loans \| loan	3,022	2,177
Unpaid principal balance	$ 2,447,402	$ 1,550,454
Fair value of loans	$ 2,479,750	$ 1,565,322
Number of loans in foreclosure \| loan	4	3
Unpaid principal balance of loans in foreclosure	$ 2,863	$ 2,251
Freddie Mac SLST
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of loans \| loan	12,444	13,605
Unpaid principal balance	$ 2,022,724	$ 2,247,771
Fair value of loans	$ 1,999,405	$ 2,221,153
Number of loans in foreclosure \| loan	305	245
Unpaid principal balance of loans in foreclosure	$ 52,319	$ 38,610
Financing Receivables, Equal to Greater than 90 Days Past Due \| Legacy Sequoia
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of loans with 90+ day delinquencies \| loan	40	52
Fair value of loans with 90+ day delinquencies	$ 14,038	$ 17,285
Financing Receivables, Equal to Greater than 90 Days Past Due \| Sequoia
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of loans with 90+ day delinquencies \| loan	30	94
Fair value of loans with 90+ day delinquencies	$ 24,438	$ 74,742
Financing Receivables, Equal to Greater than 90 Days Past Due \| Freddie Mac SLST
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of loans with 90+ day delinquencies \| loan	1,168	2,110
Fair value of loans with 90+ day delinquencies	$ 209,913	$ 389,245

Fair Value, Option, Loans Held As Assets, Aggregate Amount In Foreclosure

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Financing Receivable, Number Of Loans In Foreclosure

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Financing Receivable, Number Of Loans Past Due

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Number of mortgage loans within investment in mortgage loan on real estate by entity with substantial portion of business acquiring and holding investment real estate or interest in real estate.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 12
-Footnote 3
-Subsection 29

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-29(Footnote 3))
-URI http://asc.fasb.org/extlink&oid=120402547&loc=d3e617274-123014

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This item represents the aggregate fair value of loans held as assets that are 90 days or more past due for which the fair value option has been elected.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 28
-Subparagraph (e)(1)
-URI http://asc.fasb.org/extlink&oid=123596393&loc=d3e14064-108612

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Amount, after allowance for credit loss, of financing receivable. Excludes financing receivable covered under loss sharing agreement.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919230-210447

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3))
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=124259787&loc=d3e4428-111522

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This is the sum of principal amount outstanding for both securitized and unsecuritized loans of all types.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121570589&loc=d3e107314-111719

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