Basis of Presentation - Intangible Assets – Activity (Details) $ in Thousands	9 Months Ended
Sep. 30, 2020 USD ($)
Finite-lived Intangible Assets [Roll Forward]
Carrying Value at September 30, 2020	$ 60,737
5 Arches LLC and CoreVest
Finite-lived Intangible Assets [Roll Forward]
Carrying Value at December 31, 2019	72,789
Additions	0
Amortization Expense	(12,052)
Carrying Value at September 30, 2020	$ 60,737
Weighted Average Amortization Period (in years)	6 years
5 Arches LLC and CoreVest \| Borrower network
Finite-lived Intangible Assets [Roll Forward]
Carrying Value at December 31, 2019	$ 43,952
Additions	0
Amortization Expense	(4,854)
Carrying Value at September 30, 2020	$ 39,098
Weighted Average Amortization Period (in years)	7 years
5 Arches LLC and CoreVest \| Broker network
Finite-lived Intangible Assets [Roll Forward]
Carrying Value at December 31, 2019	$ 15,083
Additions	0
Amortization Expense	(2,715)
Carrying Value at September 30, 2020	$ 12,368
Weighted Average Amortization Period (in years)	5 years
5 Arches LLC and CoreVest \| Non-compete agreements
Finite-lived Intangible Assets [Roll Forward]
Carrying Value at December 31, 2019	$ 8,236
Additions	0
Amortization Expense	(2,375)
Carrying Value at September 30, 2020	$ 5,861
Weighted Average Amortization Period (in years)	3 years
5 Arches LLC and CoreVest \| Tradenames
Finite-lived Intangible Assets [Roll Forward]
Carrying Value at December 31, 2019	$ 3,472
Additions	0
Amortization Expense	(1,000)
Carrying Value at September 30, 2020	$ 2,472
Weighted Average Amortization Period (in years)	3 years
5 Arches LLC and CoreVest \| Developed technology
Finite-lived Intangible Assets [Roll Forward]
Carrying Value at December 31, 2019	$ 1,613
Additions	0
Amortization Expense	(675)
Carrying Value at September 30, 2020	$ 938
Weighted Average Amortization Period (in years)	2 years
5 Arches LLC and CoreVest \| Loan administration fees on existing loan assets
Finite-lived Intangible Assets [Roll Forward]
Carrying Value at December 31, 2019	$ 433
Additions	0
Amortization Expense	(433)
Carrying Value at September 30, 2020	$ 0
Weighted Average Amortization Period (in years)	1 year

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274

+ Details

Name:	us-gaap_AmortizationOfIntangibleAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 926
-SubTopic 20
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=120154696&loc=d3e54445-107959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of increase in assets, excluding financial assets, lacking physical substance with a definite life, from an acquisition.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275

+ Details

Name:	us-gaap_FinitelivedIntangibleAssetsAcquired1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_BusinessAcquisitionAxis=rwt_A5ArchesLLCAndCoreVestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=rwt_BorrowerNetworkMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=rwt_BrokerNetworkMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_NoncompeteAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_TradeNamesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_DevelopedTechnologyRightsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=rwt_ManagementFeeOnExistingAssetsUnderManagementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: