497

Exhibit Index

Exhibit No.
EX-101.INS	XBRL Instance Document
EX-101.SCH	XBRL Taxonomy Extension Schema Document
EX-101.CAL	XBRL Taxonomy Extension Calculation Linkbase
EX-101.DEF	XBRL Taxonomy Extension Definition Linkbase
EX-101.LAB	XBRL Taxonomy Extension Labels Linkbase
EX-101.PRE	XBRL Taxonomy Extension Presentation Linkbase

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Document Type	dei_DocumentType	497
Document Period End Date	dei_DocumentPeriodEndDate	Dec. 31, 2017
Registrant Name	dei_EntityRegistrantName	STATE STREET INSTITUTIONAL INVESTMENT TRUST
Central Index Key	dei_EntityCentralIndexKey	0001107414
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Sep. 24, 2018
Document Effective Date	dei_DocumentEffectiveDate	Sep. 24, 2018
Prospectus Date	rr_ProspectusDate	Apr. 30, 2018

XML

Apr. 30, 2018

State Street Institutional Investment Trust

(the “Trust”)

State Street Equity 500 Index Fund

Class A (SSSVX) Class I (SSSWX) Class K (SSSYX)

State Street Aggregate Bond Index Fund

Class A (SSFCX) Class I (SSFDX) Class K (SSFEX)

State Street Small/Mid Cap Equity Index Fund

Class A (SSMJX) Class I (SSMLX) Class K (SSMKX)

(each a “Fund”, collectively the “Funds”)

SUPPLEMENT DATED SEPTEMBER 21, 2018 TO THE PROSPECTUS AND

STATEMENT OF ADDITIONAL INFORMATION (“SAI”) EACH DATED APRIL 30, 2018

Effective immediately, the Management Fee for the State Street Aggregate Bond Index Fund will be reduced and the contractual operating expense waivers for each Fund will change to lower the Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements borne by each Fund. Accordingly, effective immediately, the Prospectus is revised as follows:

STATE STREET EQUITY 500 INDEX FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 1 of the Prospectus and “Example” on page 2 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses

(expenses that you pay each year as a percentage of the value of your investment)

	Class A			Class I			Class K
Management Fee		0.02	%		0.02	%		0.02	%
Distribution and/or Shareholder Service (12b-1) Fees		0.25	%		0.00	%		0.00	%
Other Expenses2		0.29	%		0.29	%		0.09	%
Total Annual Fund Operating Expenses		0.56	%		0.31	%		0.11	%
Less Fee Waivers and/or Expense Reimbursements3		(0.09	)%		(0.09	)%		(0.09	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.47	%		0.22	%		0.02	%

¹  A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²  Other Expenses have been restated to reflect current fees for Class A and Class I shares.

³  The Fund’s investment adviser, SSGA Funds Management, Inc. (the “Adviser” or “SSGA FM”), is contractually obligated until April 30, 2020, separately with respect to both the Fund and the Portfolio, (i) to waive up to the full amount of the advisory fee payable by the Fund/Portfolio, and/or (ii) to reimburse the Fund/Portfolio for expenses to the extent that Total Annual Fund Operating Expenses (exclusive of non-recurring account fees, extraordinary expenses, and distribution, shareholder servicing, and sub-transfer agency fees) exceed 0.02% of average daily net assets on an annual basis. This waiver and/or reimbursement may not be terminated prior to April 30, 2020 except with approval of the Fund/Portfolio’s Board of Trustees.

Example

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated, and then sell all of your Fund Shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. The Example takes into account the effect of the Fund’s contractual fee waiver and/or reimbursement through April 30, 2020. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

	1 year		3 years		5 years		10 years
Class A	$	571	$	686	$	813	$	1,181
Class I	$	23	$	91	$	165	$	384
Class K	$	2	$	26	$	53	$	132

STATE STREET AGGREGATE BOND INDEX FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 7 of the Prospectus and “Example” on pages 7-8 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses

(expenses that you pay each year as a percentage of the value of your investment)

	Class A			Class I			Class K
Management Fee2		0.025	%		0.025	%		0.025	%
Distribution and/or Shareholder Service (12b-1) Fees		0.250	%		0.000	%		0.000	%
Other Expenses3		0.430	%		0.430	%		0.230	%
Acquired Fund Fees and Expenses		0.010	%		0.010	%		0.010	%
Total Annual Fund Operating Expenses		0.715	%		0.465	%		0.265	%
Less Fee Waivers and/or Expense Reimbursements4,5		(0.240	)%		(0.240	)%		(0.240	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.475	%		0.225	%		0.025	%

¹  A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²  Management Fees have been restated to reflect current fees.

³  Other Expenses have been restated to reflect current fees for Class A and Class I shares.

⁴  The Fund’s investment adviser, SSGA Funds Management, Inc. (the “Adviser” or “SSGA FM”), is contractually obligated until April 30, 2020, separately with respect to both the Fund and the Portfolio (i) to waive up to the full amount of the advisory fee payable by the Fund/Portfolio, and/or (ii) to reimburse the Fund/Portfolio for expenses to the extent that Total Annual Fund Operating Expenses (exclusive of non-recurring account fees, extraordinary expenses, and distribution, shareholder servicing, and sub-transfer agency fees) exceed 0.025% of average daily net assets on an annual basis. This waiver and/or reimbursement may not be terminated prior to April 30, 2020 except with approval of the Fund/Portfolio’s Board of Trustees.

⁵  SSGA FM is contractually obligated to waive up to the portion of the management fee and/or expenses attributable to acquired fund fees and expenses in connection with the Portfolio’s investments in To Be Announced (“TBA”) securities. This fee waiver and/or expense reimbursement may only be terminated with approval of the Fund’s Board of Trustees.

Example

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated, and then sell all of your Fund Shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. The Example takes into account the effect of the Fund’s contractual fee waiver and/or reimbursement through April 30, 2020. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

	1 year		3 years		5 years		10 years
Class A	$	422	$	572	$	735	$	1,208
Class I	$	23	$	125	$	236	$	562
Class K	$	3	$	61	$	125	$	313

STATE STREET SMALL/MID CAP EQUITY INDEX FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 22 of the Prospectus and “Example” on page 23 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses

(expenses that you pay each year as a percentage of the value of your investment)

	Class A			Class I			Class K
Management Fee		0.030	%		0.030	%		0.030	%
Distribution and/or Shareholder Service (12b-1) Fees		0.250	%		0.000	%		0.000	%
Other Expenses2		1.150	%		1.150	%		0.950	%
Total Annual Fund Operating Expenses		1.430	%		1.180	%		0.980	%
Less Fee Waivers and/or Expense Reimbursements3		(0.935	)%		(0.935	)%		(0.935	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.495	%		0.245	%		0.045	%

¹  A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²  Other Expenses have been restated to reflect current fees for Class A and Class I shares.

³  The Fund’s investment adviser, SSGA Funds Management, Inc. (the “Adviser” or “SSGA FM”), is contractually obligated until April 30, 2020, separately with respect to both the Fund and the Portfolio (i) to waive up to the full amount of the advisory fee payable by the Fund/Portfolio, and/or (ii) to reimburse the Fund/Portfolio for expenses to the extent that Total Annual Fund Operating Expenses (exclusive of non-recurring account fees, extraordinary expenses, and any class-specific expenses, such as distribution, shareholder servicing, and sub-transfer agency fees) exceed 0.045% with respect to the fund, and .03% with respect to the Portfolio, of average daily net assets on an annual basis. This waiver and/or reimbursement may not be terminated prior to April 30, 2020 except with approval of the Fund/Portfolio’s Board of Trustees.

Example

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated, and then sell all of your Fund Shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. The Example takes into account the effect of the Fund’s contractual fee waiver and/or reimbursement through April 30, 2020. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

	1 year		3 years		5 years		10 years
Class A	$	573	$	866	$	1,181	$	2,072
Class I	$	25	$	282	$	559	$	1,348
Class K	$	5	$	219	$	450	$	1,116

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

STATE STREET INSTITUTIONAL INVESTMENT TRUST

Prospectus Date

rr_ProspectusDate

Apr. 30, 2018

Supplement [Text Block]

ssiit_SupplementTextBlock

State Street Institutional Investment Trust

(the “Trust”)

State Street Equity 500 Index Fund

Class A (SSSVX) Class I (SSSWX) Class K (SSSYX)

State Street Aggregate Bond Index Fund

Class A (SSFCX) Class I (SSFDX) Class K (SSFEX)

State Street Small/Mid Cap Equity Index Fund

Class A (SSMJX) Class I (SSMLX) Class K (SSMKX)

(each a “Fund”, collectively the “Funds”)

SUPPLEMENT DATED SEPTEMBER 21, 2018 TO THE PROSPECTUS AND

STATEMENT OF ADDITIONAL INFORMATION (“SAI”) EACH DATED APRIL 30, 2018

Effective immediately, the Management Fee for the State Street Aggregate Bond Index Fund will be reduced and the contractual operating expense waivers for each Fund will change to lower the Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements borne by each Fund. Accordingly, effective immediately, the Prospectus is revised as follows:

STATE STREET EQUITY 500 INDEX FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 1 of the Prospectus and “Example” on page 2 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses

(expenses that you pay each year as a percentage of the value of your investment)

	Class A			Class I			Class K
Management Fee		0.02	%		0.02	%		0.02	%
Distribution and/or Shareholder Service (12b-1) Fees		0.25	%		0.00	%		0.00	%
Other Expenses2		0.29	%		0.29	%		0.09	%
Total Annual Fund Operating Expenses		0.56	%		0.31	%		0.11	%
Less Fee Waivers and/or Expense Reimbursements3		(0.09	)%		(0.09	)%		(0.09	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.47	%		0.22	%		0.02	%

¹  A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²  Other Expenses have been restated to reflect current fees for Class A and Class I shares.

³  The Fund’s investment adviser, SSGA Funds Management, Inc. (the “Adviser” or “SSGA FM”), is contractually obligated until April 30, 2020, separately with respect to both the Fund and the Portfolio, (i) to waive up to the full amount of the advisory fee payable by the Fund/Portfolio, and/or (ii) to reimburse the Fund/Portfolio for expenses to the extent that Total Annual Fund Operating Expenses (exclusive of non-recurring account fees, extraordinary expenses, and distribution, shareholder servicing, and sub-transfer agency fees) exceed 0.02% of average daily net assets on an annual basis. This waiver and/or reimbursement may not be terminated prior to April 30, 2020 except with approval of the Fund/Portfolio’s Board of Trustees.

Example

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated, and then sell all of your Fund Shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. The Example takes into account the effect of the Fund’s contractual fee waiver and/or reimbursement through April 30, 2020. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

	1 year		3 years		5 years		10 years
Class A	$	571	$	686	$	813	$	1,181
Class I	$	23	$	91	$	165	$	384
Class K	$	2	$	26	$	53	$	132

STATE STREET AGGREGATE BOND INDEX FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 7 of the Prospectus and “Example” on pages 7-8 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses

(expenses that you pay each year as a percentage of the value of your investment)

	Class A			Class I			Class K
Management Fee2		0.025	%		0.025	%		0.025	%
Distribution and/or Shareholder Service (12b-1) Fees		0.250	%		0.000	%		0.000	%
Other Expenses3		0.430	%		0.430	%		0.230	%
Acquired Fund Fees and Expenses		0.010	%		0.010	%		0.010	%
Total Annual Fund Operating Expenses		0.715	%		0.465	%		0.265	%
Less Fee Waivers and/or Expense Reimbursements4,5		(0.240	)%		(0.240	)%		(0.240	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.475	%		0.225	%		0.025	%

¹  A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²  Management Fees have been restated to reflect current fees.

³  Other Expenses have been restated to reflect current fees for Class A and Class I shares.

⁴  The Fund’s investment adviser, SSGA Funds Management, Inc. (the “Adviser” or “SSGA FM”), is contractually obligated until April 30, 2020, separately with respect to both the Fund and the Portfolio (i) to waive up to the full amount of the advisory fee payable by the Fund/Portfolio, and/or (ii) to reimburse the Fund/Portfolio for expenses to the extent that Total Annual Fund Operating Expenses (exclusive of non-recurring account fees, extraordinary expenses, and distribution, shareholder servicing, and sub-transfer agency fees) exceed 0.025% of average daily net assets on an annual basis. This waiver and/or reimbursement may not be terminated prior to April 30, 2020 except with approval of the Fund/Portfolio’s Board of Trustees.

⁵  SSGA FM is contractually obligated to waive up to the portion of the management fee and/or expenses attributable to acquired fund fees and expenses in connection with the Portfolio’s investments in To Be Announced (“TBA”) securities. This fee waiver and/or expense reimbursement may only be terminated with approval of the Fund’s Board of Trustees.

Example

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated, and then sell all of your Fund Shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. The Example takes into account the effect of the Fund’s contractual fee waiver and/or reimbursement through April 30, 2020. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

	1 year		3 years		5 years		10 years
Class A	$	422	$	572	$	735	$	1,208
Class I	$	23	$	125	$	236	$	562
Class K	$	3	$	61	$	125	$	313

STATE STREET SMALL/MID CAP EQUITY INDEX FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 22 of the Prospectus and “Example” on page 23 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses

(expenses that you pay each year as a percentage of the value of your investment)

	Class A			Class I			Class K
Management Fee		0.030	%		0.030	%		0.030	%
Distribution and/or Shareholder Service (12b-1) Fees		0.250	%		0.000	%		0.000	%
Other Expenses2		1.150	%		1.150	%		0.950	%
Total Annual Fund Operating Expenses		1.430	%		1.180	%		0.980	%
Less Fee Waivers and/or Expense Reimbursements3		(0.935	)%		(0.935	)%		(0.935	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.495	%		0.245	%		0.045	%

¹  A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²  Other Expenses have been restated to reflect current fees for Class A and Class I shares.

³  The Fund’s investment adviser, SSGA Funds Management, Inc. (the “Adviser” or “SSGA FM”), is contractually obligated until April 30, 2020, separately with respect to both the Fund and the Portfolio (i) to waive up to the full amount of the advisory fee payable by the Fund/Portfolio, and/or (ii) to reimburse the Fund/Portfolio for expenses to the extent that Total Annual Fund Operating Expenses (exclusive of non-recurring account fees, extraordinary expenses, and any class-specific expenses, such as distribution, shareholder servicing, and sub-transfer agency fees) exceed 0.045% with respect to the fund, and .03% with respect to the Portfolio, of average daily net assets on an annual basis. This waiver and/or reimbursement may not be terminated prior to April 30, 2020 except with approval of the Fund/Portfolio’s Board of Trustees.

Example

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated, and then sell all of your Fund Shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. The Example takes into account the effect of the Fund’s contractual fee waiver and/or reimbursement through April 30, 2020. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

	1 year		3 years		5 years		10 years
Class A	$	573	$	866	$	1,181	$	2,072
Class I	$	25	$	282	$	559	$	1,348
Class K	$	5	$	219	$	450	$	1,116

State Street Equity 500 Index Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

ssiit_SupplementTextBlock

State Street Institutional Investment Trust

(the “Trust”)

State Street Equity 500 Index Fund

Class A (SSSVX) Class I (SSSWX) Class K (SSSYX)

(each a “Fund”, collectively the “Funds”)

SUPPLEMENT DATED SEPTEMBER 21, 2018 TO THE PROSPECTUS AND

STATEMENT OF ADDITIONAL INFORMATION (“SAI”) EACH DATED APRIL 30, 2018

Effective immediately, the Management Fee for the State Street Aggregate Bond Index Fund will be reduced and the contractual operating expense waivers for each Fund will change to lower the Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements borne by each Fund. Accordingly, effective immediately, the Prospectus is revised as follows:

STATE STREET EQUITY 500 INDEX FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 1 of the Prospectus and “Example” on page 2 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses

(expenses that you pay each year as a percentage of the value of your investment)

	Class A			Class I			Class K
Management Fee		0.02	%		0.02	%		0.02	%
Distribution and/or Shareholder Service (12b-1) Fees		0.25	%		0.00	%		0.00	%
Other Expenses2		0.29	%		0.29	%		0.09	%
Total Annual Fund Operating Expenses		0.56	%		0.31	%		0.11	%
Less Fee Waivers and/or Expense Reimbursements3		(0.09	)%		(0.09	)%		(0.09	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.47	%		0.22	%		0.02	%

¹  A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²  Other Expenses have been restated to reflect current fees for Class A and Class I shares.

³  The Fund’s investment adviser, SSGA Funds Management, Inc. (the “Adviser” or “SSGA FM”), is contractually obligated until April 30, 2020, separately with respect to both the Fund and the Portfolio, (i) to waive up to the full amount of the advisory fee payable by the Fund/Portfolio, and/or (ii) to reimburse the Fund/Portfolio for expenses to the extent that Total Annual Fund Operating Expenses (exclusive of non-recurring account fees, extraordinary expenses, and distribution, shareholder servicing, and sub-transfer agency fees) exceed 0.02% of average daily net assets on an annual basis. This waiver and/or reimbursement may not be terminated prior to April 30, 2020 except with approval of the Fund/Portfolio’s Board of Trustees.

Example

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated, and then sell all of your Fund Shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. The Example takes into account the effect of the Fund’s contractual fee waiver and/or reimbursement through April 30, 2020. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

	1 year		3 years		5 years		10 years
Class A	$	571	$	686	$	813	$	1,181
Class I	$	23	$	91	$	165	$	384
Class K	$	2	$	26	$	53	$	132

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<b>Annual Fund Operating Expenses</b><br/><br/>(expenses that you pay each year as a percentage of the value of your investment)

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

April 30, 2020

Expenses Deferred Charges [Text Block]

rr_ExpensesDeferredChargesTextBlock

A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

Expenses Restated to Reflect Current [Text]

rr_ExpensesRestatedToReflectCurrent

Other Expenses have been restated to reflect current fees for Class A and Class I shares.

Expense Example [Heading]

rr_ExpenseExampleHeading

<b>Example</b>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated, and then sell all of your Fund Shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. The Example takes into account the effect of the Fund’s contractual fee waiver and/or reimbursement through April 30, 2020. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

State Street Equity 500 Index Fund | Class A

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

0.02%

Distribution and/or Shareholder Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

0.25%

Other Expenses

rr_OtherExpensesOverAssets

0.29%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

0.56%

Less Fee Waivers and/or Expense Reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.09%)

^[2]

Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements

rr_NetExpensesOverAssets

0.47%

1 year

rr_ExpenseExampleYear01

$ 571

3 years

rr_ExpenseExampleYear03

686

5 years

rr_ExpenseExampleYear05

813

10 years

rr_ExpenseExampleYear10

$ 1,181

State Street Equity 500 Index Fund | Class I

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

0.02%

Distribution and/or Shareholder Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

none

Other Expenses

rr_OtherExpensesOverAssets

0.29%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

0.31%

Less Fee Waivers and/or Expense Reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.09%)

^[2]

Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements

rr_NetExpensesOverAssets

0.22%

1 year

rr_ExpenseExampleYear01

$ 23

3 years

rr_ExpenseExampleYear03

91

5 years

rr_ExpenseExampleYear05

165

10 years

rr_ExpenseExampleYear10

$ 384

State Street Equity 500 Index Fund | Class K

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

0.02%

Distribution and/or Shareholder Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

none

Other Expenses

rr_OtherExpensesOverAssets

0.09%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

0.11%

Less Fee Waivers and/or Expense Reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.09%)

^[2]

Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements

rr_NetExpensesOverAssets

0.02%

1 year

rr_ExpenseExampleYear01

$ 2

3 years

rr_ExpenseExampleYear03

26

5 years

rr_ExpenseExampleYear05

53

10 years

rr_ExpenseExampleYear10

$ 132

State Street Aggregate Bond Index Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

ssiit_SupplementTextBlock

State Street Institutional Investment Trust

(the “Trust”)

State Street Aggregate Bond Index Fund

Class A (SSFCX) Class I (SSFDX) Class K (SSFEX)

(each a “Fund”, collectively the “Funds”)

SUPPLEMENT DATED SEPTEMBER 21, 2018 TO THE PROSPECTUS AND

STATEMENT OF ADDITIONAL INFORMATION (“SAI”) EACH DATED APRIL 30, 2018

Effective immediately, the Management Fee for the State Street Aggregate Bond Index Fund will be reduced and the contractual operating expense waivers for each Fund will change to lower the Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements borne by each Fund. Accordingly, effective immediately, the Prospectus is revised as follows:

STATE STREET AGGREGATE BOND INDEX FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 7 of the Prospectus and “Example” on pages 7-8 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses

(expenses that you pay each year as a percentage of the value of your investment)

	Class A			Class I			Class K
Management Fee2		0.025	%		0.025	%		0.025	%
Distribution and/or Shareholder Service (12b-1) Fees		0.250	%		0.000	%		0.000	%
Other Expenses3		0.430	%		0.430	%		0.230	%
Acquired Fund Fees and Expenses		0.010	%		0.010	%		0.010	%
Total Annual Fund Operating Expenses		0.715	%		0.465	%		0.265	%
Less Fee Waivers and/or Expense Reimbursements4,5		(0.240	)%		(0.240	)%		(0.240	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.475	%		0.225	%		0.025	%

¹  A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²  Management Fees have been restated to reflect current fees.

³  Other Expenses have been restated to reflect current fees for Class A and Class I shares.

⁴  The Fund’s investment adviser, SSGA Funds Management, Inc. (the “Adviser” or “SSGA FM”), is contractually obligated until April 30, 2020, separately with respect to both the Fund and the Portfolio (i) to waive up to the full amount of the advisory fee payable by the Fund/Portfolio, and/or (ii) to reimburse the Fund/Portfolio for expenses to the extent that Total Annual Fund Operating Expenses (exclusive of non-recurring account fees, extraordinary expenses, and distribution, shareholder servicing, and sub-transfer agency fees) exceed 0.025% of average daily net assets on an annual basis. This waiver and/or reimbursement may not be terminated prior to April 30, 2020 except with approval of the Fund/Portfolio’s Board of Trustees.

⁵  SSGA FM is contractually obligated to waive up to the portion of the management fee and/or expenses attributable to acquired fund fees and expenses in connection with the Portfolio’s investments in To Be Announced (“TBA”) securities. This fee waiver and/or expense reimbursement may only be terminated with approval of the Fund’s Board of Trustees.

Example

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated, and then sell all of your Fund Shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. The Example takes into account the effect of the Fund’s contractual fee waiver and/or reimbursement through April 30, 2020. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

	1 year		3 years		5 years		10 years
Class A	$	422	$	572	$	735	$	1,208
Class I	$	23	$	125	$	236	$	562
Class K	$	3	$	61	$	125	$	313

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<b>Annual Fund Operating Expenses</b><br/><br/>(expenses that you pay each year as a percentage of the value of your investment)

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

April 30, 2020

Expenses Deferred Charges [Text Block]

rr_ExpensesDeferredChargesTextBlock

A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

Expenses Restated to Reflect Current [Text]

rr_ExpensesRestatedToReflectCurrent

Management Fees have been restated to reflect current fees.<br/><br/>Other Expenses have been restated to reflect current fees for Class A and Class I shares.

Expense Example [Heading]

rr_ExpenseExampleHeading

<b>Example</b>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated, and then sell all of your Fund Shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. The Example takes into account the effect of the Fund’s contractual fee waiver and/or reimbursement through April 30, 2020. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

State Street Aggregate Bond Index Fund | Class A

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

0.025%

^[3]

Distribution and/or Shareholder Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

0.25%

Other Expenses

rr_OtherExpensesOverAssets

0.43%

^[1]

Acquired Fund Fees and Expenses

rr_AcquiredFundFeesAndExpensesOverAssets

0.01%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

0.715%

Less Fee Waivers and/or Expense Reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.24%)

^[4],[5]

Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements

rr_NetExpensesOverAssets

0.475%

1 year

rr_ExpenseExampleYear01

$ 422

3 years

rr_ExpenseExampleYear03

572

5 years

rr_ExpenseExampleYear05

735

10 years

rr_ExpenseExampleYear10

$ 1,208

State Street Aggregate Bond Index Fund | Class I

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

0.025%

^[3]

Distribution and/or Shareholder Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

none

Other Expenses

rr_OtherExpensesOverAssets

0.43%

^[1]

Acquired Fund Fees and Expenses

rr_AcquiredFundFeesAndExpensesOverAssets

0.01%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

0.465%

Less Fee Waivers and/or Expense Reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.24%)

^[4],[5]

Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements

rr_NetExpensesOverAssets

0.225%

1 year

rr_ExpenseExampleYear01

$ 23

3 years

rr_ExpenseExampleYear03

125

5 years

rr_ExpenseExampleYear05

236

10 years

rr_ExpenseExampleYear10

$ 562

State Street Aggregate Bond Index Fund | Class K

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

0.025%

^[3]

Distribution and/or Shareholder Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

none

Other Expenses

rr_OtherExpensesOverAssets

0.23%

^[1]

Acquired Fund Fees and Expenses

rr_AcquiredFundFeesAndExpensesOverAssets

0.01%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

0.265%

Less Fee Waivers and/or Expense Reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.24%)

^[4],[5]

Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements

rr_NetExpensesOverAssets

0.025%

1 year

rr_ExpenseExampleYear01

$ 3

3 years

rr_ExpenseExampleYear03

61

5 years

rr_ExpenseExampleYear05

125

10 years

rr_ExpenseExampleYear10

$ 313

STATE STREET SMALL/MID CAP EQUITY INDEX FUND

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

ssiit_SupplementTextBlock

State Street Institutional Investment Trust

(the “Trust”)

State Street Small/Mid Cap Equity Index Fund

Class A (SSMJX) Class I (SSMLX) Class K (SSMKX)

(each a “Fund”, collectively the “Funds”)

SUPPLEMENT DATED SEPTEMBER 21, 2018 TO THE PROSPECTUS AND

STATEMENT OF ADDITIONAL INFORMATION (“SAI”) EACH DATED APRIL 30, 2018

Effective immediately, the Management Fee for the State Street Aggregate Bond Index Fund will be reduced and the contractual operating expense waivers for each Fund will change to lower the Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements borne by each Fund. Accordingly, effective immediately, the Prospectus is revised as follows:

STATE STREET SMALL/MID CAP EQUITY INDEX FUND

The sub-sections within the Fund Summary entitled “Annual Fund Operating Expenses” on page 22 of the Prospectus and “Example” on page 23 of the Prospectus are each deleted in their entirety and replaced in each instance with the following:

Annual Fund Operating Expenses

(expenses that you pay each year as a percentage of the value of your investment)

	Class A			Class I			Class K
Management Fee		0.030	%		0.030	%		0.030	%
Distribution and/or Shareholder Service (12b-1) Fees		0.250	%		0.000	%		0.000	%
Other Expenses2		1.150	%		1.150	%		0.950	%
Total Annual Fund Operating Expenses		1.430	%		1.180	%		0.980	%
Less Fee Waivers and/or Expense Reimbursements3		(0.935	)%		(0.935	)%		(0.935	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements		0.495	%		0.245	%		0.045	%

¹  A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

²  Other Expenses have been restated to reflect current fees for Class A and Class I shares.

³  The Fund’s investment adviser, SSGA Funds Management, Inc. (the “Adviser” or “SSGA FM”), is contractually obligated until April 30, 2020, separately with respect to both the Fund and the Portfolio (i) to waive up to the full amount of the advisory fee payable by the Fund/Portfolio, and/or (ii) to reimburse the Fund/Portfolio for expenses to the extent that Total Annual Fund Operating Expenses (exclusive of non-recurring account fees, extraordinary expenses, and any class-specific expenses, such as distribution, shareholder servicing, and sub-transfer agency fees) exceed 0.045% with respect to the fund, and .03% with respect to the Portfolio, of average daily net assets on an annual basis. This waiver and/or reimbursement may not be terminated prior to April 30, 2020 except with approval of the Fund/Portfolio’s Board of Trustees.

Example

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated, and then sell all of your Fund Shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. The Example takes into account the effect of the Fund’s contractual fee waiver and/or reimbursement through April 30, 2020. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

	1 year		3 years		5 years		10 years
Class A	$	573	$	866	$	1,181	$	2,072
Class I	$	25	$	282	$	559	$	1,348
Class K	$	5	$	219	$	450	$	1,116

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<b>Annual Fund Operating Expenses</b><br/><br/>(expenses that you pay each year as a percentage of the value of your investment)

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

April 30, 2020

Expenses Deferred Charges [Text Block]

rr_ExpensesDeferredChargesTextBlock

A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

Expenses Restated to Reflect Current [Text]

rr_ExpensesRestatedToReflectCurrent

Other Expenses have been restated to reflect current fees for Class A and Class I shares.

Expense Example [Heading]

rr_ExpenseExampleHeading

<b>Example</b>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated, and then sell all of your Fund Shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. The Example takes into account the effect of the Fund’s contractual fee waiver and/or reimbursement through April 30, 2020. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

STATE STREET SMALL/MID CAP EQUITY INDEX FUND | Class A

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

0.03%

Distribution and/or Shareholder Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

0.25%

Other Expenses

rr_OtherExpensesOverAssets

1.15%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.43%

Less Fee Waivers and/or Expense Reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.935%)

^[6]

Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements

rr_NetExpensesOverAssets

0.495%

1 year

rr_ExpenseExampleYear01

$ 573

3 years

rr_ExpenseExampleYear03

866

5 years

rr_ExpenseExampleYear05

1,181

10 years

rr_ExpenseExampleYear10

$ 2,072

STATE STREET SMALL/MID CAP EQUITY INDEX FUND | Class I

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

0.03%

Distribution and/or Shareholder Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

none

Other Expenses

rr_OtherExpensesOverAssets

1.15%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.18%

Less Fee Waivers and/or Expense Reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.935%)

^[6]

Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements

rr_NetExpensesOverAssets

0.245%

1 year

rr_ExpenseExampleYear01

$ 25

3 years

rr_ExpenseExampleYear03

282

5 years

rr_ExpenseExampleYear05

559

10 years

rr_ExpenseExampleYear10

$ 1,348

STATE STREET SMALL/MID CAP EQUITY INDEX FUND | Class K

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

0.03%

Distribution and/or Shareholder Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

none

Other Expenses

rr_OtherExpensesOverAssets

0.95%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

0.98%

Less Fee Waivers and/or Expense Reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.935%)

^[6]

Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements

rr_NetExpensesOverAssets

0.045%

1 year

rr_ExpenseExampleYear01

$ 5

3 years

rr_ExpenseExampleYear03

219

5 years

rr_ExpenseExampleYear05

450

10 years

rr_ExpenseExampleYear10

$ 1,116

[1] Other Expenses have been restated to reflect current fees for Class A and Class I shares.

[2] The Fund’s investment adviser, SSGA Funds Management, Inc. (the “Adviser” or “SSGA FM”), is contractually obligated until April 30, 2020, separately with respect to both the Fund and the Portfolio, (i) to waive up to the full amount of the advisory fee payable by the Fund/Portfolio, and/or (ii) to reimburse the Fund/Portfolio for expenses to the extent that Total Annual Fund Operating Expenses (exclusive of non-recurring account fees, extraordinary expenses, and distribution, shareholder servicing, and sub-transfer agency fees) exceed 0.02% of average daily net assets on an annual basis. This waiver and/or reimbursement may not be terminated prior to April 30, 2020 except with approval of the Fund/Portfolio’s Board of Trustees.

[3] Management Fees have been restated to reflect current fees.

[4] SSGA FM is contractually obligated to waive up to the portion of the management fee and/or expenses attributable to acquired fund fees and expenses in connection with the Portfolio’s investments in To Be Announced (“TBA”) securities. This fee waiver and/or expense reimbursement may only be terminated with approval of the Fund’s Board of Trustees.

[5] The Fund’s investment adviser, SSGA Funds Management, Inc. (the “Adviser” or “SSGA FM”), is contractually obligated until April 30, 2020, separately with respect to both the Fund and the Portfolio (i) to waive up to the full amount of the advisory fee payable by the Fund/Portfolio, and/or (ii) to reimburse the Fund/Portfolio for expenses to the extent that Total Annual Fund Operating Expenses (exclusive of non-recurring account fees, extraordinary expenses, and distribution, shareholder servicing, and sub-transfer agency fees) exceed 0.025% of average daily net assets on an annual basis. This waiver and/or reimbursement may not be terminated prior to April 30, 2020 except with approval of the Fund/Portfolio’s Board of Trustees.

[6] The Fund’s investment adviser, SSGA Funds Management, Inc. (the “Adviser” or “SSGA FM”), is contractually obligated until April 30, 2020, separately with respect to both the Fund and the Portfolio (i) to waive up to the full amount of the advisory fee payable by the Fund/Portfolio, and/or (ii) to reimburse the Fund/Portfolio for expenses to the extent that Total Annual Fund Operating Expenses (exclusive of non-recurring account fees, extraordinary expenses, and any class-specific expenses, such as distribution, shareholder servicing, and sub-transfer agency fees) exceed 0.045% with respect to the fund, and .03% with respect to the Portfolio, of average daily net assets on an annual basis. This waiver and/or reimbursement may not be terminated prior to April 30, 2020 except with approval of the Fund/Portfolio’s Board of Trustees.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	STATE STREET INSTITUTIONAL INVESTMENT TRUST
Prospectus Date	rr_ProspectusDate	Apr. 30, 2018
Document Creation Date	dei_DocumentCreationDate	Sep. 24, 2018