Name:	star_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetNetChangeInUnrealizedGainLossOnInvestments
Namespace Prefix:	star_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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

No definition available.

Name:	star_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetNetRealizedGainLossOnInvestments
Namespace Prefix:	star_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Amount of proceeds from principal repayments 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.

No definition available.

Name:	star_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetProceedsFromPrincipalRepaymentsOfInvestments
Namespace Prefix:	star_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Amount of Lien status change 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.

No definition available.

Name:	star_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLienStatusChangeOfInvestments
Namespace Prefix:	star_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Amount of investments on net of original issue discount on nonoperating securities.

No definition available.

Name:	star_InvestmentNetOfOriginalIssueDiscount
Namespace Prefix:	star_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Number of existing funds into which company made investment.

No definition available.

Name:	star_NumberOfExistingFunds
Namespace Prefix:	star_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Number of existing portfolio companies where company made investment.

No definition available.

Name:	star_NumberOfExistingPortfolioCompanies
Namespace Prefix:	star_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Number of new portfolio companies where company made investment.

No definition available.

Name:	star_NumberOfNewPortfolioCompanies
Namespace Prefix:	star_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

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

No definition available.

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

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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

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

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.

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

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

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

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

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

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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-SubTopic 10
-Topic 820
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

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

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

Name:	us-gaap_InvestmentTypeAxis=star_FirstLienSeniorSecuredLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_InvestmentTypeAxis=star_SecondLienSeniorSecuredLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_InvestmentTypeAxis=star_SeniorUnsecuredNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_InvestmentTypeAxis=us-gaap_PreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_InvestmentTypeAxis=star_WarrantsAndOtherEquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_InvestmentTypeAxis=star_FundInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_InvestmentTypeAxis=star_InvestmentPortfolio4Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_InvestmentTypeAxis=star_InvestmentPortfolio7Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_InvestmentTypeAxis=star_InvestmentPortfolio1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: