Commitments and Contingencies - Additional Information (Details)			1 Months Ended	3 Months Ended				9 Months Ended
		Sep. 30, 2020 USD ($) lease	May 31, 2020 USD ($)	Sep. 30, 2020 USD ($) lease	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2018 partnership	Sep. 30, 2020 USD ($) lease repurchase_request loan	Sep. 30, 2019 USD ($) repurchase_request loan	Dec. 31, 2019 USD ($)	Mar. 01, 2019 USD ($)	May 31, 2018 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Loss Contingencies [Line Items]
Lessee, number of leases | lease		7		7				7
Present value of remaining lease payments		$ 18,175,000		$ 18,175,000				$ 18,175,000
Operating lease expense								$ 3,000,000	$ 2,000,000
Number of new office leases | lease								3
Lease liability		$ 15,123,000		$ 15,123,000				$ 15,123,000		$ 13,443,000
Operating lease liability, statement of financial position [Extensible List]		us-gaap:AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent		us-gaap:AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent				us-gaap:AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent
Right-of-use asset		$ 13,487,000		$ 13,487,000				$ 13,487,000		11,866,000
Operating lease right-of-use asset, statement of financial position [Extensible List]		us-gaap:OtherAssets		us-gaap:OtherAssets				us-gaap:OtherAssets
Weighted average remaining lease term		7 years		7 years				7 years
Discount rate		5.00%		5.00%				5.00%
Derivative liabilities	[1]	$ 1,612,000		$ 1,612,000				$ 1,612,000		163,424,000
Number of partnerships, committed to fund | partnership							2
Contingent consideration		0		0				0		28,484,000
Other income related to risk sharing agreement				1,200,000		$ 905,000		3,146,000	$ 2,351,000
Guarantee obligations		11,264,000		11,264,000				11,264,000		14,009,000
Guarantee obligations, credit reserve		5,000,000		5,000,000				5,000,000
Special purpose entities assets		47,000,000		47,000,000				47,000,000		48,000,000
Special purpose entities liabilities		11,000,000		11,000,000				11,000,000		14,000,000
Residential repurchase reserve		8,565,000		8,565,000				$ 8,565,000		4,268,000
Number of residential repurchase requests (in repurchase requests) | repurchase_request								8	10
Number of loans repurchased | loan								0	0
Residential loans repurchase provision (reversal)								$ 4,000,000	$ (200,000)
Aggregate amount of loss contingency reserves		2,000,000		2,000,000				2,000,000
Damages sought			$ 2,000,000
Residential Loan Seller Demands
Loss Contingencies [Line Items]
Expenses from litigation recorded				4,000,000				9,000,000
Aggregate amount of accrual		6,500,000		6,500,000				6,500,000
Minimum | Residential Loan Seller Demands
Loss Contingencies [Line Items]
Reasonably possible losses estimated		0		0				0
Maximum | Residential Loan Seller Demands
Loss Contingencies [Line Items]
Reasonably possible losses estimated		1,500,000		1,500,000				1,500,000
Residential Loans
Loss Contingencies [Line Items]
Loan principal		102,921,000		102,921,000				102,921,000		524,928,000
Financing Receivables, Equal to Greater than 90 Days Past Due | Residential Loans
Loss Contingencies [Line Items]
Balance of loans 90 days or more delinquent		2,356,000		2,356,000				2,356,000		$ 747,000
Other income
Loss Contingencies [Line Items]
Other income related to risk sharing agreement				1,000,000		1,000,000		3,000,000	2,000,000
Mortgage banking and investment activities
Loss Contingencies [Line Items]
Market valuation losses related to these investments				(300,000)		$ (100,000)		(900,000)	$ (200,000)
Guarantee Obligations
Loss Contingencies [Line Items]
Original unpaid balance of loans subject to risk sharing agreements													$ 3,190,000,000	$ 3,190,000,000
Potential future payments on loans		44,000,000		44,000,000				44,000,000
Loan principal		$ 1,120,000,000		$ 1,120,000,000				$ 1,120,000,000
Weighted average original FICO score		758		758				758
Weighted average original loan-to-value (LTV)		75.00%		75.00%				75.00%
Guarantee Obligations | Residential Loans
Loss Contingencies [Line Items]
Fair value of loans, in foreclosure		$ 0		$ 0				$ 0
Guarantee Obligations | Financing Receivables, Equal to Greater than 90 Days Past Due
Loss Contingencies [Line Items]
Balance of loans 90 days or more delinquent		42,000,000		42,000,000				42,000,000
Commitment To Fund Residential Bridge Loan
Loss Contingencies [Line Items]
Commitments to fund temporary advances		225,000,000		225,000,000				225,000,000
Derivative liabilities		1,000,000		1,000,000				1,000,000
Commitment To Fund Temporary Advances On Residential Bridge Loans
Loss Contingencies [Line Items]
Commitments to fund temporary advances		15,000,000		15,000,000				15,000,000
5 Arches
Loss Contingencies [Line Items]
Contingent consideration maximum amount												$ 29,000,000
Cash payment for contingent consideration liability					$ 11,000,000
Restricted stock awards					$ 3,000,000
Contingent consideration		14,000,000		14,000,000				14,000,000			$ 25,000,000
Shared home appreciation options
Loss Contingencies [Line Items]
Payments to acquire investments		47,000,000
Shared home appreciation options | Commitment To Fund Investment
Loss Contingencies [Line Items]
Commitments to fund temporary advances		$ 3,000,000		$ 3,000,000				$ 3,000,000

[1] Our consolidated balance sheets include assets of consolidated variable interest entities (“VIEs”) that can only be used to settle obligations of these VIEs and liabilities of consolidated VIEs for which creditors do not have recourse to Redwood Trust, Inc. or its affiliates. At September 30, 2020 and December 31, 2019, assets of consolidated VIEs totaled $8,197,095 and $11,931,869, respectively. At September 30, 2020 and December 31, 2019, liabilities of consolidated VIEs totaled $7,238,047 and $10,717,072, respectively. See Note 4 for further discussion.