Secured and Unsecured Debt of the Operating Partnership - Unsecured Revolving Credit Facility and Term Loan Facility (Details) - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Borrowings on unsecured debt	$ 120,000,000	$ 0
Terms of the Credit Facility
Outstanding borrowings	330,000,000		$ 0
Kilroy Realty L.P. [Member]
Debt Instrument [Line Items]
Borrowings on unsecured debt	120,000,000	$ 0
Terms of the Credit Facility
Outstanding borrowings	330,000,000		0
Unamortized deferred financing costs	13,300,000
Kilroy Realty L.P. [Member] \| Revolving credit facility [Member]
Debt Instrument [Line Items]
Contingent additional borrowings	600,000,000
Terms of the Credit Facility
Outstanding borrowings	330,000,000		0
Remaining borrowing capacity	420,000,000		750,000,000
Total borrowing capacity	$ 750,000,000		$ 750,000,000
Interest rate (percent)	3.23%		2.56%
Facility fee-annual rate (percent)	0.20%		0.20%
Maturity date	Jul. 01, 2022		Jul. 01, 2022
Unamortized deferred financing costs	$ 5,000,000		$ 6,000,000
London Interbank Offered Rate (LIBOR) [Member] \| Kilroy Realty L.P. [Member] \| Revolving credit facility [Member]
Terms of the Credit Facility
Variable rate (percent)	1.00%		1.00%
$150 Million Term Loan Facility [Member] \| Kilroy Realty L.P. [Member] \| Line of credit [Member]
Debt Instrument [Line Items]
Increase in borrowings	$ 30,000,000
Borrowings on unsecured debt	120,000,000
Terms of the Credit Facility
Outstanding borrowings	150,000,000		$ 0
Remaining borrowing capacity	$ 0		150,000,000
Total borrowing capacity			$ 150,000,000
Interest rate (percent)	3.23%		2.66%
Undawn facility fee-annual rate (percent)	0.20%		0.20%
Maturity date	Jul. 01, 2022		Jul. 01, 2022
Unamortized deferred financing costs	$ 1,000,000		$ 1,200,000
$150 Million Term Loan Facility [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Kilroy Realty L.P. [Member] \| Line of credit [Member]
Terms of the Credit Facility
Variable rate (percent)	1.10%		1.10%

Debt Instrument, Contingent Additional Borrowings

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

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- 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.

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

+ References

No definition available.

+ Details

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- Definition

Date the credit facility terminates, in CCYY-MM-DD format.

+ References

+ Details

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- Definition

Amount of increase (decrease) of the credit facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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- Definition

The effective interest rate at the end of the reporting period.

+ References

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Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

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Period Type:	instant

- Definition

The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

+ References

No definition available.

+ Details

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- Definition

The cash inflow from borrowings supported by a written promise to pay an obligation that is uncollateralized (where debt is not backed by the pledge of collateral).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3255-108585

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- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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