Leases - Summary of Future Minimum Lease Payments Under Non-Cancelable Leases (Detail) $ in Thousands	Sep. 30, 2025 USD ($)
Leases [Abstract]
2025 (remainder)	$ 2,844
2026	7,590
2027	8,273
2028	6,547
2029	6,623
Thereafter	44,223
Total	76,100
2025 (remainder)	(342)
2026	(16,809)
2027	0
2028	0
2029	0
Thereafter	0
Total	(17,151)
2025 (remainder)	2,502
2026	(9,219)
2027	8,273
2028	6,547
2029	6,623
Thereafter	44,223
Total	58,949
Less imputed interest	(19,661)
Present value of lease liabilities	$ 39,288

Lease payments allowance.

No definition available.

Name:	forr_LeasePaymentsAllowance
Namespace Prefix:	forr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Lease payments net undiscounted cash flows.

+ References

No definition available.

+ Details

Name:	forr_LeasePaymentsNetUndiscountedCashFlows
Namespace Prefix:	forr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Lessee operating lease liability payments due after year four.

+ References

No definition available.

+ Details

Name:	forr_LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFour
Namespace Prefix:	forr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Net undiscounted cash flows due next twelve months.

+ References

No definition available.

+ Details

Name:	forr_NetUndiscountedCashFlowsDueNextTwelveMonths
Namespace Prefix:	forr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Net undiscounted cash flows payments due after year four.

+ References

No definition available.

+ Details

Name:	forr_NetUndiscountedCashFlowsPaymentsDueAfterYearFour
Namespace Prefix:	forr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Net undiscounted cash flows payments due year four.

+ References

No definition available.

+ Details

Name:	forr_NetUndiscountedCashFlowsPaymentsDueYearFour
Namespace Prefix:	forr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Net undiscounted cash flows payments due year three.

+ References

No definition available.

+ Details

Name:	forr_NetUndiscountedCashFlowsPaymentsDueYearThree
Namespace Prefix:	forr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Net undiscounted cash flows payments due year two

+ References

No definition available.

+ Details

Name:	forr_NetUndiscountedCashFlowsPaymentsDueYearTwo
Namespace Prefix:	forr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Net undiscounted cash flows remainder of fiscal year.

+ References

No definition available.

+ Details

Name:	forr_NetUndiscountedCashFlowsRemainderOfFiscalYear
Namespace Prefix:	forr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Tenant improvement allowance due next twelve months.

+ References

No definition available.

+ Details

Name:	forr_TenantImprovementAllowanceDueNextTwelveMonths
Namespace Prefix:	forr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Tenant improvement allowance payments due after year four.

+ References

No definition available.

+ Details

Name:	forr_TenantImprovementAllowancePaymentsDueAfterYearFour
Namespace Prefix:	forr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Tenant improvement allowance payments due year four.

+ References

No definition available.

+ Details

Name:	forr_TenantImprovementAllowancePaymentsDueYearFour
Namespace Prefix:	forr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Tenant improvement allowance payments due year three.

+ References

No definition available.

+ Details

Name:	forr_TenantImprovementAllowancePaymentsDueYearThree
Namespace Prefix:	forr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Tenant improvement allowance payments due year two.

+ References

No definition available.

+ Details

Name:	forr_TenantImprovementAllowancePaymentsDueYearTwo
Namespace Prefix:	forr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Tenant improvement allowance remainder of fiscal year

+ References

No definition available.

+ Details

Name:	forr_TenantImprovementAllowanceRemainderOfFiscalYear
Namespace Prefix:	forr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Amount of lessee's undiscounted obligation for lease payment for operating lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 235
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477314/942-235-S99-2

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480738/235-10-S50-4

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-3

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478964/842-20-50-6

+ Details

Name:	us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of lessee's undiscounted obligation for lease payment for operating lease to be paid in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

Name:	us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of lessee's undiscounted obligation for lease payment for operating lease to be paid in fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

Name:	us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFour
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of lessee's undiscounted obligation for lease payment for operating lease to be paid in third fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

Name:	us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearThree
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of lessee's undiscounted obligation for lease payment for operating lease to be paid in second fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

Name:	us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearTwo
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of lessee's undiscounted obligation for lease payment for operating lease having initial or remaining lease term in excess of one year to be paid in remainder of current fiscal year.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478964/842-20-50-6

+ Details

Name:	us-gaap_LesseeOperatingLeaseLiabilityPaymentsRemainderOfFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of lessee's undiscounted obligation for lease payments in excess of discounted obligation for lease payments for operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478964/842-20-50-6

+ Details

Name:	us-gaap_LesseeOperatingLeaseLiabilityUndiscountedExcessAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Present value of lessee's discounted obligation for lease payments from operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479041/842-20-45-1

+ Details

Name:	us-gaap_OperatingLeaseLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant