| Label | Element | Value |
|---|---|---|
| Risk Return Abstract | rr_RiskReturnAbstract | |
| Document Type | dei_DocumentType | 485BPOS |
| Period End Date | dei_DocumentPeriodEndDate | Dec. 31, 2024 |
| Registrant Name | dei_EntityRegistrantName | STATE STREET INSTITUTIONAL INVESTMENT TRUST |
| CIK | dei_EntityCentralIndexKey | 0001107414 |
| Amendment Flag | dei_AmendmentFlag | false |
| Document Creation Date | dei_DocumentCreationDate | Apr. 25, 2025 |
| Document Effective Date | dei_DocumentEffectiveDate | Apr. 30, 2025 |
| Prospectus Date | rr_ProspectusDate | Apr. 30, 2025 |
| Entity Inv Company Type | dei_EntityInvCompanyType | N-1A |
| X | ||||||||||
- Definition Boolean flag that is true when the XBRL content amends previously-filed or accepted submission. No definition available.
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| X | ||||||||||
- Definition The date the document was made available and submitted, in YYYY-MM-DD format. The date of submission, date of acceptance by the recipient, and the document effective date are all potentially different. No definition available.
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| X | ||||||||||
- Definition The date when a document, upon receipt and acceptance, becomes officially effective, in YYYY-MM-DD format. Usually it is a system-assigned date time value, but it may be declared by the submitter in some cases. No definition available.
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| X | ||||||||||
- Definition For the EDGAR submission types of Form 8-K: the date of the report, the date of the earliest event reported; for the EDGAR submission types of Form N-1A: the filing date; for all other submission types: the end of the reporting or transition period. The format of the date is YYYY-MM-DD. No definition available.
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| X | ||||||||||
- Definition The type of document being provided (such as 10-K, 10-Q, 485BPOS, etc). The document type is limited to the same value as the supporting SEC submission type, or the word 'Other'. No definition available.
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| X | ||||||||||
- Definition A unique 10-digit SEC-issued value to identify entities that have filed disclosures with the SEC. It is commonly abbreviated as CIK. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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| X | ||||||||||
- Definition One of: N-1A (Mutual Fund), N-1 (Open-End Separate Account with No Variable Annuities), N-2 (Closed-End Investment Company), N-3 (Separate Account Registered as Open-End Management Investment Company), N-4 (Variable Annuity UIT Separate Account), N-5 (Small Business Investment Company), N-6 (Variable Life UIT Separate Account), S-1 or S-3 (Face Amount Certificate Company), S-6 (UIT, Non-Insurance Product). Reference 1: http://www.xbrl.org/2003/role/presentationRef
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| X | ||||||||||
- Definition The exact name of the entity filing the report as specified in its charter, which is required by forms filed with the SEC. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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| X | ||||||||||
- Definition The date of the prospectus. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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| X | ||||||||||
- References No definition available.
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