Acquisitions (Details Textuals) (USD $)	12 Months Ended							0 Months Ended					12 Months Ended				0 Months Ended		12 Months Ended				0 Months Ended												12 Months Ended								12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2012 Middlefield, Mountain View, CA [Member]	May 09, 2012 Middlefield, Mountain View, CA [Member]	Sep. 28, 2012 6121 Sunset Boulevard, Los Angeles, CA [Member]	Dec. 04, 2012 Fairchild Drive, 331, Mountain View, CA [Member]	Dec. 31, 2012 Mathilda, 555, Sunnyvale, CA [Member]		Dec. 31, 2012 Real Estate Acquisitions [Member] building	Dec. 31, 2011 Real Estate Acquisitions [Member] building	Dec. 31, 2011 601 108th Avenue N.E Bellevue, WA [Member]		Dec. 31, 2012 Fremont Lake Union Center Seattle WA [Member] building		Dec. 17, 2012 N. Mathilda Avenue 599 and 555, Sunnyvale, CA [Member]		Dec. 31, 2012 In-Place Leases [Member]	Dec. 31, 2012 Deferred Leasing Costs [Member]	Dec. 31, 2012 Below Market Leases [Member]	Dec. 31, 2012 Above Market Leases [Member]	Jul. 31, 2012 Kilroy Realty, L.P. [Member] 6255 Sunset Boulevard, Los Angeles, CA [Member]	Dec. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member]	Jun. 30, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 5.09% Mortgage Payable due August 2015 [Member] [Member] building	Oct. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 5.57% Mortgage Note Payable due February 2016 [Member] building	Apr. 30, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] 4.94% Mortgage Payable due April 15, 2015 [Member]	Jul. 24, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 6.37% Mortgage Payable due April 2013 [Member] building	Jul. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 5.23% Mortgage Payable due January 2016 [Member] building	Dec. 31, 2012 Office Properties [Member] Real Estate Acquisitions [Member] building	Dec. 31, 2011 Office Properties [Member] 601 108th Avenue N.E Bellevue, WA [Member] building	Dec. 31, 2012 Office Properties [Member] 6255 Sunset Boulevard, Los Angeles, CA [Member] building		Jul. 31, 2012 Office Properties [Member] Kilroy Realty, L.P. [Member] 6255 Sunset Boulevard, Los Angeles, CA [Member]	Dec. 31, 2012 Maximum [Member]	Dec. 31, 2011 Undeveloped Land and Construction in Progress [Member] In-Place Leases [Member]	Dec. 31, 2011 Undeveloped Land and Construction in Progress [Member] Deferred Leasing Costs [Member]	Dec. 31, 2011 Undeveloped Land and Construction in Progress [Member] Above Market Leases [Member]	Dec. 31, 2011 Deferred Leasing Costs and Acquisition-Related Intangible Assets [Member] Below Market Leases [Member]	Dec. 31, 2011 Deferred Leasing Costs and Acquisition-Related Intangible Assets [Member] Above Market Leases [Member]	Dec. 31, 2011 Above Market Leases [Member]	Dec. 31, 2012 Above Market Leases [Member]	Dec. 31, 2012 Below Market Leases [Member]	Dec. 31, 2012 Ten Year Ground Lease Extension Option [Member] Extensions	Dec. 31, 2012 Forty-Five Year Ground Lease Extension Option [Member] Extensions
Business Acquisition [Line Items]
Land under Purchase Options, Not Recorded																																			$ 27,500,000
Number of buildings											14	10			2	[1]									2	1		1	1	24	1	1	[2]
Cash and cash equivalents	3,973,000														4,000,000
Business Acquisition, Percentage of Voting Interests Acquired						100.00%
Secured debt, net (Notes 5 and 12)																									34,000,000	40,700,000	30,000,000	83,600,000	53,900,000
Initial premium on outstanding amount of secured debt																								8,900,000	1,700,000	2,700,000	1,000,000	1,400,000	3,100,000
Business Acquisition, Liabilities Assumed, Excluded from Purchase Price								2,100,000									11,800,000																	4,700,000
Business Acquisition, Cost of Acquired Entity, Cash Paid					74,500,000			18,900,000									137,600,000	[3],[4]
Business Combination, Consideration Transferred, Development Costs Paid								2,900,000									2,400,000
BusinessAcquisitionLeasingCommissionsAssumed					9,500,000
Business Acquisition, Cost of Acquired Entity, Liabilities Incurred							1,100,000
Purchase Price Of Each Acquisition As Percentage Of Companys Assets																																			5.00%
Purchase Price of all Acquisitions as Percentage of Company Assets																																			10.00%
Number of extension options																																												3	1
Ten Year Ground Lease Extension Option	10 years
Forty-Five Year Ground Lease Extension Option	45 years
Assumed noncancellable ground lease expiration													Nov. 30, 2093
Deferred leasing costs and acquisition-related intangible assets	59,513,000	[5]	42,630,000	[6]									13,790,000	[6]					43,400,000	14,200,000	500,000	1,400,000														27,400,000	8,500,000	6,800,000	18,700,000	5,200,000
Weighted average amortization period in years																			4 years 8 months 0 days	3 years 5 months 6 days	59 years 7 months 6 days	3 years 9 months 12 days														3 years 9 months 0 days	2 years 6 months 0 days	4 years 6 months 0 days	3 years 11 months 0 days	82 years 6 months 0 days
Deferred revenue and acquisition-related intangible liabilities	35,020,000	[7]	23,940,000	[8]									12,850,000	[8]																												1,100,000	33,900,000
AcquiredFiniteLivedIntangibleLiabilitiesWeightedAverageUsefulLife																																									29 years 7 months 6 days		6 years 6 months 12 days
Business Acquisition, Purchase Price Allocation, Current Assets, Prepaid Expense and Other Assets															200,000
Accounts payable, accrued expenses, and other liabilities	$ 6,348,000		$ 4,515,000						$ 23,071,000	[9]			$ 2,380,000		$ 600,000
Issuance of common units in connection with an operating property acquisition	118,372																						118,372
Share Price																							$ 47.34

[1] We acquired these properties through the acquisition of the ownership interest of the bankruptcy remote LLC that owned the properties. In connection with this acquisition we also acquired cash of approximately $4.0 million and other assets of approximately $0.2 million and we assumed current liabilities of approximately $0.6 million and secured debt with an outstanding principal balance of $34.0 million and a premium of $1.7 million as a result of recording the debt at fair value at the acquisition date (see Note 7).

[2] As part of the consideration for this transaction, we issued 118,372 common units of the Operating Partnership valued at $47.34 per unit, which was the Company's closing stock price on the NYSE on the acquisition date. In connection with this acquisition we also assumed secured debt with an outstanding principal balance of $53.9 million and a premium of $3.1 million as a result of recording this debt at fair value on the acquisition date (see Note 7). We also assumed $4.7 million of accrued liabilities in connection with this acquisition that are not included in the purchase price above.

[3] Excludes acquisition-related costs and non-lease related accrued liabilities assumed. Includes assumed unpaid leasing commissions and tenant improvements.

[4] This operating property was acquired in connection with the purchase of the 555 N. Mathilda Ave. development property discussed in further detail in the "Development and Redevelopment Project Site" section of this footnote, for a total purchase price of $137.6 million. The acquisition of both the operating property and the development site in a single transaction constituted our third largest acquisition of 2012.

[5] Represents in-place leases (approximately $43.4 million with a weighted average amortization period of 4.7 years), above-market leases (approximately $1.4 million with a weighted average amortization period of 3.8 years), leasing commissions (approximately $14.2 million with a weighted average amortization period of 3.4 years), and a below-market ground lease obligation (approximately $0.5 million with a weighted average amortization period of 59.6 years).

[6] Represents below-market leases (approximately $18.7 million with a weighted average amortization period of 3.9 years) and an above-market ground lease obligation (approximately $5.2 million with a weighted average amortization period of 82.5 years), under which we are the lessee.

[7] Represents below-market leases (approximately $33.9 million with a weighted average amortization period of 6.5 years) and an above-market ground lease obligation (approximately $1.1 million with a weighted average amortization period of 29.6 years).

[8] Represents the mortgage loan, which includes an unamortized premium of approximately $1.0 million at the date of acquisition, assumed in connection with the properties acquired in April 2011 (see Note 7).

[9] In connection with this acquisition, restricted cash is being held in escrow to pay for potential environmental costs and contingent development costs. Any unused amounts will be released to the seller.