DERIVATIVE INSTRUMENTS (Notional Amount) (Details) - Interest Rate Swaps $ in Thousands	3 Months Ended
Mar. 31, 2018 USD ($) investment	Dec. 31, 2017 USD ($) investment
Derivative instruments designated as hedging instruments
Derivative [Line Items]
Number of Instruments \| investment	2	2
Notional Amount \| $	$ 118,400	$ 118,400
Weighted-Average Fix Pay Rate	1.53%
Weighted-Average Remaining Term in Years	3 months 12 days
Derivative instruments designated as hedging instruments \| Minimum \| One-month LIBOR
Derivative [Line Items]
Reference Rate	1.41%
Derivative instruments designated as hedging instruments \| Maximum \| One-month LIBOR
Derivative [Line Items]
Reference Rate	1.68%
Derivative instruments not designated as hedging instruments
Derivative [Line Items]
Number of Instruments \| investment	15	16
Notional Amount \| $	$ 1,209,506	$ 1,209,643
Weighted-Average Fix Pay Rate	1.97%
Weighted-Average Remaining Term in Years	3 years 4 months 6 days
Derivative instruments not designated as hedging instruments \| Minimum \| One-month LIBOR
Derivative [Line Items]
Reference Rate	1.39%
Derivative instruments not designated as hedging instruments \| Maximum \| One-month LIBOR
Derivative [Line Items]
Reference Rate	2.37%

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Derivative Instruments, Weighted-Average Fix Pay Rate

+ References

No definition available.

+ Details

Name:	kbsriii_DerivativeInstrumentsWeightedAverageFixPayRate
Namespace Prefix:	kbsriii_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Derivative Instruments, Weighted-Average Remaining Term in Years

+ References

No definition available.

+ Details

Name:	kbsriii_DerivativeInstrumentsWeightedAverageRemainingTerminYears
Namespace Prefix:	kbsriii_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeFixedInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

The number of derivative instruments of a particular group held by the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNumberOfInstrumentsHeld
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: