TENANT ORIGINATION AND ABSORPTION COSTS, ABOVE-MARKET LEASE ASSETS AND BELOW-MARKET LEASE LIABILITIES (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025
Tenant Origination and Absorption Costs, Above-Market Lease Assets and Below-Market Lease Liabilities [Abstract]
Tenant origination and absorption costs, cost	$ 22,307		$ 22,309
Tenant origination and absorption costs, accumulated amortization	(18,776)		(18,340)
Tenant origination and absorption costs, net amount	3,531		3,969
Tenant origination and absorption costs, amortization	(438)	$ (602)
Above-market lease assets, cost	873		873
Above-market lease assets, accumulated amortization	(838)		(821)
Above market leases, net amount	35		52
Above-market lease assets, amortization	(17)	(17)
Below-market lease liabilities, cost	(2,809)		(2,809)
Below-market lease liabilities, accumulated amortization	2,644		2,579
Below-market lease liabilities, net amount	(165)		$ (230)
Below-market lease liabilities, amortization	$ 65	$ 97

Above-market leases, accumulated amortization.

No definition available.

Name:	kbsriii_AboveMarketLeasesAccumulatedAmortization
Namespace Prefix:	kbsriii_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Above-market leases, amortization expense.

+ References

No definition available.

+ Details

Name:	kbsriii_AboveMarketLeasesAmortizationExpense
Namespace Prefix:	kbsriii_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Above-market leases, gross.

+ References

No definition available.

+ Details

Name:	kbsriii_AboveMarketLeasesGross
Namespace Prefix:	kbsriii_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents an asset associated with the acquisition of an off-market lease when the terms of the lease are favorable to the market terms for the lease at the date of acquisition.

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No definition available.

+ Details

Name:	kbsriii_AboveMarketLeasesNet
Namespace Prefix:	kbsriii_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Off-market lease, unfavorable, amortization expense.

+ References

No definition available.

+ Details

Name:	kbsriii_OffMarketLeaseUnfavorableAmortizationExpense
Namespace Prefix:	kbsriii_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Tenant Origination and Absorption Costs, Above-Market Lease Assets and Below-Market Lease Liabilities [Abstract]

+ References

No definition available.

+ Details

Name:	kbsriii_TenantOriginationAndAbsorptionCostsAboveMarketLeaseAssetsAndBelowMarketLeaseLiabilitiesAbstract
Namespace Prefix:	kbsriii_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Tenant origination and absorption costs, accumulated Amortization

+ References

No definition available.

+ Details

Name:	kbsriii_TenantOriginationAndAbsorptionCostsAccumulatedAmortization
Namespace Prefix:	kbsriii_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Tenant origination and absorption costs, amortization expense.

+ References

No definition available.

+ Details

Name:	kbsriii_TenantOriginationAndAbsorptionCostsAmortizationExpense
Namespace Prefix:	kbsriii_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the net amount of tenant origination and absorption costs. Origination costs include estimates of costs avoided associated with leasing the property, including tenant allowance and improvements and leasing commissions. Absorption costs relate to the value of opportunity costs calculated using the contractual amounts paid pursuant to in-place leases over market absorption period for a similar lease.

+ References

No definition available.

+ Details

Name:	kbsriii_TenantOriginationAndAbsorptionCostsGross
Namespace Prefix:	kbsriii_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Tenant origination and absorption costs, net.

+ References

No definition available.

+ Details

Name:	kbsriii_TenantOriginationAndAbsorptionCostsNet
Namespace Prefix:	kbsriii_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated amortization of leases acquired as part of a real property acquisition at below market lease rate with a finite life.

+ References

No definition available.

+ Details

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

- Definition

Amount before amortization of leases acquired as part of a real property acquisition at below market lease rate with a finite life.

+ References

No definition available.

+ Details

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

- Definition

Amount after amortization of leases acquired as part of a real property acquisition at below market lease rate with a finite life.

+ References

No definition available.

+ Details

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