Schedule III - Real Estate and Accumulated Depreciation - Narrative (Details) $ in Thousands	12 Months Ended
Dec. 31, 2018 USD ($) ft² property_units	Dec. 31, 2017 USD ($)
Real Estate and Accumulated Depreciation [Line Items]
Initial cost, encumbrances	$ 335,811
Rentable square feet \| ft²	13,232,580
Debt premium	$ 800
Aggregate cost of property for federal income tax purposes	7,000,000
Aggregate cost of property held in a VIE	$ 100,000
Building [Member]
Real Estate and Accumulated Depreciation [Line Items]
Depreciation life	35 years
Tenant Improvements [Member] \| Minimum [Member]
Real Estate and Accumulated Depreciation [Line Items]
Property depreciable lives	1 year
Tenant Improvements [Member] \| Maximum [Member]
Real Estate and Accumulated Depreciation [Line Items]
Property depreciable lives	20 years
Office Building [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial cost, encumbrances	$ 335,811
Rentable square feet \| ft²	13,232,580
1550 N. El Centro Avenue, Los Angeles, CA [Member] \| Office Building [Member]
Real Estate and Accumulated Depreciation [Line Items]
Depreciation life	35 years
Number of building units \| property_units	200
12100 W. Olympic Blvd., Los Angeles, CA [Member] \| Office Building [Member]
Real Estate and Accumulated Depreciation [Line Items]
Depreciation life	35 years
Initial cost, encumbrances	$ 170,000
Rentable square feet \| ft²	152,048
2100/2110 Colorado Ave., Santa Monica, CA [Member] \| Office Building [Member]
Real Estate and Accumulated Depreciation [Line Items]
Depreciation life	35 years
Initial cost, encumbrances	$ 91,332
Rentable square feet \| ft²	102,864
320 Westlake Avenue North, WA[Member] \| Office Building [Member]
Real Estate and Accumulated Depreciation [Line Items]
Depreciation life	35 years
Initial cost, encumbrances	$ 74,479
Rentable square feet \| ft²	184,644
100 Hooper Street, San Francisco, CA [Member] \| Office Building [Member]
Real Estate and Accumulated Depreciation [Line Items]
Depreciation life	35 years
Kilroy Realty L.P. [Member]
Real Estate and Accumulated Depreciation [Line Items]
Unamortized debt issuance expense	$ 17,400
Kilroy Realty L.P. [Member] \| Secured debt [Member]
Real Estate and Accumulated Depreciation [Line Items]
Debt premium	800
Unamortized debt issuance expense	$ 1,039	$ 1,175
Development Projects in Lease-Up [Member] \| 100 Hooper Street, San Francisco, CA [Member] \| Office Building [Member]
Real Estate and Accumulated Depreciation [Line Items]
Rentable square feet \| ft²	400,000

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SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Held in VIE

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Amount of lien or mortgage outstanding on real estate investment properties for entities with a substantial portion of business acquiring and holding investment real estate.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 12
-Paragraph Column B
-Subsection 28

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Topic 970
-SubTopic 360
-Section S99
-Paragraph 1
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Cost basis of real estate investments for federal income tax purposes for entities with a substantial portion of business acquiring and holding investment real estate.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Name Regulation S-X (SX)
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-Subsection 28

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Net rentable area for properties owned.

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Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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Useful life on which depreciation was computed for entities with a substantial portion of business acquiring and holding investment real estate, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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