Secured and Unsecured Debt of the Operating Partnership (Details) (USD $)								9 Months Ended				1 Months Ended	9 Months Ended				1 Months Ended	9 Months Ended					9 Months Ended				9 Months Ended				9 Months Ended						9 Months Ended				9 Months Ended				9 Months Ended				9 Months Ended				12 Months Ended
	Sep. 30, 2012 NumberofBuildings		Dec. 31, 2011		Sep. 30, 2012 Kilroy Realty, L.P. [Member]	Dec. 31, 2011 Kilroy Realty, L.P. [Member]	Sep. 30, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member]	Sep. 30, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] Four Point Two Seven Percent Mortgage Payable due Feb One Two Thousand Eighteen Member		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] Four Point Two Seven Percent Mortgage Payable due Feb One Two Thousand Eighteen Member		Jun. 30, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 4.48% Mortgage Payable due July 1, 2027 [Member] Years Y	Sep. 30, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 4.48% Mortgage Payable due July 1, 2027 [Member]		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] 4.48% Mortgage Payable due July 1, 2027 [Member]		Jul. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] Six Point Thirty Seven Percent Mortgage Payable due April First Two Thousand Thirteen [Member] Y	Sep. 30, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] Six Point Thirty Seven Percent Mortgage Payable due April First Two Thousand Thirteen [Member]		Oct. 05, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] Six Point Thirty Seven Percent Mortgage Payable due April First Two Thousand Thirteen [Member]	Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] Six Point Thirty Seven Percent Mortgage Payable due April First Two Thousand Thirteen [Member]		Sep. 30, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] Five Point Five Seven Percent Mortgage Payable due August One Two Thousand Twelve [Member]		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] Five Point Five Seven Percent Mortgage Payable due August One Two Thousand Twelve [Member]		Sep. 30, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] Six Point Five One Percent Mortgage Payable due February One Two Thousand Seventeen [Member]		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] Six Point Five One Percent Mortgage Payable due February One Two Thousand Seventeen [Member]		Sep. 30, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] Five Point Twenty Three Percent Mortgage Payable due January First Two Thousand Sixteen [Member]		Aug. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] Five Point Twenty Three Percent Mortgage Payable due January First Two Thousand Sixteen [Member] NumberofBuildings	Jul. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] Five Point Twenty Three Percent Mortgage Payable due January First Two Thousand Sixteen [Member]	Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] Five Point Twenty Three Percent Mortgage Payable due January First Two Thousand Sixteen [Member]		Sep. 30, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 5.09% Mortgage Payable due August 7, 2015 [Member] NumberofBuildings		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] 5.09% Mortgage Payable due August 7, 2015 [Member]		Sep. 30, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] Four Point Nine four Percent Mortgage Payable due April Fifteen Two Thousand Fifteen Member		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] Four Point Nine four Percent Mortgage Payable due April Fifteen Two Thousand Fifteen Member		Sep. 30, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] Four Point Nine Five Percent Mortgage Payable due August One Two Thousand Twelve [Member]		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] Four Point Nine Five Percent Mortgage Payable due August One Two Thousand Twelve [Member]		Sep. 30, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] Seven Point Fifteen Percent Mortgage Payable due May One Two Thousand Seventeen [Member]		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] Seven Point Fifteen Percent Mortgage Payable due May One Two Thousand Seventeen [Member]		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] Municipal Bonds [Member]		Sep. 30, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] Municipal Bonds [Member]
Debt Instrument [Line Items]
Annual Stated Interest Rate								4.27%	[1]				4.48%	[1],[2],[3]				6.37%	[1],[2],[4]						5.57%	[1],[5]			6.51%	[1]	5.23%	[1],[2],[6]					5.09%	[1],[2],[7]					4.94%	[1],[2]			4.95%	[1],[5]			7.15%	[1]
GAAP Effective Rate								4.27%	[1],[8]				4.48%	[1],[2],[3],[8]				3.55%	[1],[2],[4],[8]				5.57%	[1],[5],[8]			6.51%	[1],[8]			3.50%	[1],[2],[6],[8]					3.50%	[1],[2],[7],[8]			4.00%	[1],[2],[8]			4.95%	[1],[5],[8]			7.15%	[1],[8]
Maturity Date								Feb. 01, 2018					Jul. 01, 2027	[2],[3]				Apr. 01, 2013	[2],[4]				Aug. 01, 2012	[5]			Feb. 01, 2017				Jan. 01, 2016	[2],[6]					Aug. 07, 2015	[2],[7]			Apr. 15, 2015	[2]			Aug. 01, 2012	[5]			May 01, 2017
Secured Debt	$ 520,867,000	[9]	$ 351,825,000	[9]	$ 520,867,000	$ 351,825,000		$ 135,000,000	[9]	$ 135,000,000	[9]		$ 97,000,000	[2],[3],[9]	$ 0	[2],[3],[9]		$ 84,238,000	[2],[4],[9]		$ 0	[2],[4],[9]	$ 0	[5],[9]	$ 71,517,000	[5],[9]	$ 68,844,000	[9]	$ 69,507,000	[9]	$ 56,730,000	[2],[6],[9]			$ 0	[2],[6],[9]	$ 35,512,000	[2],[7],[9]	$ 0	[2],[7],[9]	$ 29,258,000	[2],[9]	$ 30,191,000	[2],[9]	$ 0	[5],[9]	$ 29,754,000	[5],[9]	$ 11,745,000	[9]	$ 13,294,000	[9]	$ 2,562,000	[10],[9]	$ 2,540,000	[10],[9]
Amortization period of debt												30					30
Period of interest only payments												3
Principal amount of secured debt																				83,600,000														53,900,000			34,000,000
Debt premium							$ 6,200,000													$ 1,400,000														$ 3,100,000			$ 1,700,000
Number of buildings	12																																1				2
Interest rates range, minimum																																																					4.74%
Interest rates range, maximum																																																					6.20%

[1] All interest rates presented are fixed-rate interest rates.

[2] The secured debt and the related properties that secure the debt are held in a special purpose entity and the properties are not available to satisfy the debts and other obligations of the Company or the Operating Partnership.

[3] In June 2012, we obtained a mortgage loan that is secured by office properties located at 2211 Michelson in Irvine, California and 2100 and 2110 Colorado Avenue in Santa Monica, California and requires monthly principal and interest payments based on a 30-year amortization period with an initial three years of interest only payments.

[4] In July 2012, in connection with the acquisition of one office building in Bellevue, Washington, we assumed the mortgage loan that is secured by the project. The assumed mortgage loan had a principal balance of $83.6 million at the acquisition date and was recorded at fair value on the date of the acquisition resulting in a premium of approximately $1.4 million. This premium will be accreted on a straight-line basis, which approximates the effective interest method, as a reduction to interest expense. The loan requires monthly principal and interest payments based on a 30-year amortization period.

[5] In May 2012, we repaid these loans prior to the stated maturity.

[6] In July 2012, in connection with the acquisition of one office building in Los Angeles, California, we assumed the mortgage loan that is secured by the project. The assumed mortgage had a principal balance of $53.9 million at the acquisition date and was recorded at fair value on the date of the acquisition resulting in a premium of approximately $3.1 million. This premium will be accreted on a straight-line basis, which approximates the effective interest method, as a reduction to interest expense from the acquisition date through the maturity date of the mortgage loan. The loan requires monthly principal and interest payments based on a 30-year amortization period.

[7] In June 2012, in connection with the acquisition of two office buildings in Seattle, Washington, we assumed a mortgage loan that is secured by the project. The assumed mortgage loan had a principal balance of $34.0 million at the acquisition date and was recorded at fair value at the date of acquisition resulting in an initial premium ofapproximately $1.7 million. This premium will be accreted on a straight−line basis, which approximates the effective interest method, as a reduction to interest expense from theacquisition date through the maturity date of the mortgage loan.

[8] This represents the rate at which interest expense is recorded for financial reporting purposes, which reflects the amortization of discounts/premiums, excluding debt issuance costs.

[9] Amounts reported include the amounts of unamortized debt premiums and discounts for the periods presented.

[10] The public facility bonds (the “Bonds”), the proceeds from which were used to finance infrastructure improvements on one of the Company’s undeveloped land parcels, were issued in February 2008 by the City of Carlsbad. The Bonds have annual maturities from September 1, 2013 through September 1, 2038, with interest rates ranging from 4.74% to 6.20%. Principal and interest payments for the Bonds will be charged through the assessment of special property taxes.