Fair Value - Level III (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Total realized and unrealized gains (losses):
Included in earnings: Net accretion			$ 7,857	$ 9,583
Level III
Changes in financial assets classified as Level III
Balance at the beginning of the period	$ 56,032,952	$ 48,041,073	52,931,064	49,904,651
Total realized and unrealized gains (losses):
Included in earnings: Change in fair value / gain on sale	152,072	(1,456,318)	490,578	(3,243,895)
Included in earnings: Net accretion	2,535	2,622	5,038	5,441
Included in OCI	(79)	1,023	(466)	2,186
Purchases / Originations	911,938	634,896	1,652,234	912,155
Sales	(367,795)	(215,940)	(932,725)	(482,572)
Issuances	(25,045)		(58,723)	(7,948)
Cash repayments / receipts	(50,773)	(117,550)	(77,165)	(181,547)
Transfers into Level III	(594,399)	(160,071)	(1,259,791)	(690,959)
Transfers out of Level III	294,227	(85,918)	430,819	122,340
Consolidations of VIEs	819,529	725,189	3,996,285	1,815,070
Deconsolidations of VIEs	(255,129)	(145,801)	(257,115)	(931,717)
Balance at the end of the period	56,920,033	47,223,205	56,920,033	47,223,205
Amount of total (losses) gains included in earnings attributable to assets still held at period end	137,657	(1,466,598)	466,726	(3,259,799)
Loans held-for-sale \| Level III
Changes in financial assets classified as Level III
Balance at the beginning of the period	841,687	723,733	671,282	745,743
Total realized and unrealized gains (losses):
Included in earnings: Change in fair value / gain on sale	21,891	14,833	33,157	22,633
Purchases / Originations	911,938	633,433	1,652,234	910,692
Sales	(367,045)	(215,133)	(928,747)	(481,765)
Cash repayments / receipts	(36,073)	(64,097)	(55,528)	(104,534)
Transfers out of Level III		(195,510)		(195,510)
Balance at the end of the period	1,372,398	897,259	1,372,398	897,259
Amount of total (losses) gains included in earnings attributable to assets still held at period end	5,547	2,071	5,145	1,482
RMBS \| Level III
Changes in financial assets classified as Level III
Balance at the beginning of the period	204,835	240,853	209,079	247,021
Total realized and unrealized gains (losses):
Included in earnings: Change in fair value / gain on sale		141		141
Included in earnings: Net accretion	2,535	2,622	5,038	5,441
Included in OCI	(79)	1,023	(466)	2,186
Sales		(807)		(807)
Cash repayments / receipts	(6,417)	(8,036)	(12,777)	(18,186)
Balance at the end of the period	200,874	235,796	200,874	235,796
Amount of total (losses) gains included in earnings attributable to assets still held at period end	2,535	2,623	5,038	5,388
CMBS \| Level III
Changes in financial assets classified as Level III
Balance at the beginning of the period	38,335	23,969	25,228	24,191
Total realized and unrealized gains (losses):
Included in earnings: Change in fair value / gain on sale	1,016	(542)	721	13
Purchases / Originations		1,463		1,463
Sales	(750)		(3,978)
Cash repayments / receipts	(5,402)	(240)	(5,590)	(1,017)
Transfers into Level III			5,350
Deconsolidations of VIEs	1,084		12,552
Balance at the end of the period	34,283	24,650	34,283	24,650
Amount of total (losses) gains included in earnings attributable to assets still held at period end	410	(542)	(157)	13
Domestic Servicing Rights \| Level III
Changes in financial assets classified as Level III
Balance at the beginning of the period	19,790	24,945	20,557	30,759
Total realized and unrealized gains (losses):
Included in earnings: Change in fair value / gain on sale	(916)	(2,203)	(1,683)	(8,017)
Balance at the end of the period	18,874	22,742	18,874	22,742
Amount of total (losses) gains included in earnings attributable to assets still held at period end	(916)	(2,203)	(1,683)	(8,017)
VIE Assets \| Level III
Changes in financial assets classified as Level III
Balance at the beginning of the period	56,974,864	49,233,307	53,446,364	51,045,874
Total realized and unrealized gains (losses):
Included in earnings: Change in fair value / gain on sale	126,589	(1,766,507)	420,934	(3,793,715)
Consolidations of VIEs	824,070	725,189	4,104,135	1,815,070
Deconsolidations of VIEs	(257,917)	(147,116)	(303,827)	(1,022,356)
Balance at the end of the period	57,667,606	48,044,873	57,667,606	48,044,873
Amount of total (losses) gains included in earnings attributable to assets still held at period end	126,589	(1,766,507)	420,934	(3,793,715)
VIE liabilities \| Level III
Changes in financial assets classified as Level III
Balance at the beginning of the period	(2,046,559)	(2,205,734)	(1,441,446)	(2,188,937)
Total realized and unrealized gains (losses):
Included in earnings: Change in fair value / gain on sale	3,492	297,960	37,449	535,050
Issuances	(25,045)		(58,723)	(7,948)
Cash repayments / receipts	(2,881)	(45,177)	(3,270)	(57,810)
Transfers into Level III	(594,399)	(160,071)	(1,265,141)	(690,959)
Transfers out of Level III	294,227	109,592	430,819	317,850
Consolidations of VIEs	(4,541)		(107,850)
Deconsolidations of VIEs	1,704	1,315	34,160	90,639
Balance at the end of the period	(2,374,002)	(2,002,115)	(2,374,002)	(2,002,115)
Amount of total (losses) gains included in earnings attributable to assets still held at period end	$ 3,492	$ 297,960	$ 37,449	$ 535,050

No definition available.

Name:	stwd_FairValueAssetsMeasuredOnRecurringBasisRealizedAndUnrealizedGainsAndLossesAbstract
Namespace Prefix:	stwd_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the change due to the consolidations of Variable Interest Entity (VIE) that have taken place during the period in relation to the assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ References

No definition available.

+ Details

Name:	stwd_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetConsolidationsOfVariableInterestEntity
Namespace Prefix:	stwd_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the change due to the deconsolidations of Variable Interest Entity (VIE) that have taken place during the period in relation to the assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ References

No definition available.

+ Details

Name:	stwd_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetDeconsolidationsOfVariableInterestEntity
Namespace Prefix:	stwd_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This item represents the amount of the total realized and unrealized gains or losses for the period which are included in the statement of income (or changes in net assets) in change in fair value; the fair value of which assets was or is measured on a recurring basis using significant unobservable inputs (Level 3).

+ References

No definition available.

+ Details

Name:	stwd_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInChangeInFairValue
Namespace Prefix:	stwd_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) recognized on the income statement for a financial instrument classified as an asset measured using unobservable inputs, still held at the end of the period.

+ References

No definition available.

+ Details

Name:	stwd_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarningsAttributableToAssetsStillHeldAtEndOfPeriod
Namespace Prefix:	stwd_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

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.

+ References

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 http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

+ Details

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

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

- Definition

Amount of gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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

- Definition

Amount of issuances of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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

- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

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

- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

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

- Definition

Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

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

- Definition

Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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