Mortgage-Backed Securities and Other Securities - Financing Receivables (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2023	Mar. 31, 2022
Components of interest income
Net (Premium Amortization/Amortization Basis) Discount Accretion	$ (260)	$ (2,228)
Interest Income	40,857	35,642
Agency RMBS
Components of interest income
Coupon Interest	0	8
Net (Premium Amortization/Amortization Basis) Discount Accretion	3	(4)
Interest Income	3	4
Non-Agency CMBS
Components of interest income
Coupon Interest	1,956	2,972
Net (Premium Amortization/Amortization Basis) Discount Accretion	317	(402)
Interest Income	2,273	2,570
Non-Agency RMBS
Components of interest income
Coupon Interest	589	545
Net (Premium Amortization/Amortization Basis) Discount Accretion	(155)	(15)
Interest Income	434	530
Other securities
Components of interest income
Coupon Interest	840	888
Net (Premium Amortization/Amortization Basis) Discount Accretion	(102)	(234)
Interest Income	738	654
Total
Components of interest income
Coupon Interest	3,385	4,413
Net (Premium Amortization/Amortization Basis) Discount Accretion	63	(655)
Interest Income	$ 3,448	$ 3,758

The sum of the periodic adjustments of the differences between securities' face values and purchase prices that are charged against earnings. This is called accretion if the security was purchased at a discount and amortization if it was purchased at premium. As a noncash item, this element is an adjustment 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
-URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3602-108585

+ Details

Name:	us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of operating interest income, including, but not limited to, amortization and accretion of premiums and discounts on securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(e))
-URI https://asc.fasb.org/extlink&oid=126953954&loc=SL114868664-224227

+ Details

Name:	us-gaap_InterestIncomeOperating
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

Amount before accretion (amortization) of purchase discount (premium) of interest income on operating securities.

+ References

No definition available.

+ Details

Name:	wmc_InvestmentIncomeInterestOperating
Namespace Prefix:	wmc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_FinancialInstrumentAxis=wmc_ResidentialMortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=wmc_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=wmc_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=wmc_OtherSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: