Derivatives and Hedging Activity - Designated and Non-Designated Hedges (Details) € in Thousands, £ in Thousands, $ in Thousands	Mar. 31, 2020 GBP (£) item	Mar. 31, 2020 EUR (€) item	Mar. 31, 2020 USD ($) item
Derivatives
Number of contracts	396	396	396
Foreign exchange contracts \| EUR \| Short
Derivatives
Number of contracts	230	230	230
Aggregate notional amount \| €		€ 215,677
Foreign exchange contracts \| AUD \| Short
Derivatives
Number of contracts	7	7	7
Aggregate notional amount \| $			$ 49,793
Foreign exchange contracts \| GBP \| Long
Derivatives
Number of contracts	2	2	2
Aggregate notional amount \| £	£ 2,414
Foreign exchange contracts \| GBP \| Short
Derivatives
Number of contracts	94	94	94
Aggregate notional amount \| $			$ 383,837
Interest rate swaps - Paying fixed rates \| USD
Derivatives
Number of contracts	40	40	40
Aggregate notional amount \| $			$ 1,493,447
Interest rate swaps - Receiving fixed rates \| USD
Derivatives
Number of contracts	2	2	2
Aggregate notional amount \| $			$ 970,000
Interest Rate Swap Guarantees \| USD
Derivatives
Number of contracts	6	6	6
Aggregate notional amount \| $			$ 392,607
Interest rate caps \| USD
Derivatives
Number of contracts	10	10	10
Aggregate notional amount \| $			$ 717,777
Credit spread instrument \| USD
Derivatives
Number of contracts	4	4	4
Aggregate notional amount \| $			$ 69,000
Forward loan purchase commitments \| USD
Derivatives
Number of contracts	1	1	1
Aggregate notional amount \| $			$ 17,988

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

+ Details

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

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of derivative instruments of a particular group held by the entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959

+ Details

Name:	us-gaap_DerivativeNumberOfInstrumentsHeld
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeForwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CurrencyAxis=currency_EUR
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PositionAxis=us-gaap_ShortMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CurrencyAxis=currency_AUD
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CurrencyAxis=currency_GBP
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PositionAxis=us-gaap_LongMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=stwd_InterestRateSwapPayingFixedRatesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CurrencyAxis=currency_USD
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=stwd_InterestRateSwapReceivingFixedRatesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=stwd_InterestRateSwapGuaranteesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateCapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_CreditIndexProductMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForwardContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: