Secured and Unsecured Debt of the Operating Partnership Secured and Unsecured Debt of the Operating Partnership - Secured Debt (Details) (USD $)											12 Months Ended				1 Months Ended	12 Months Ended				1 Months Ended	12 Months Ended					12 Months Ended				12 Months Ended				1 Months Ended	12 Months Ended				12 Months Ended					12 Months Ended					12 Months Ended					12 Months Ended				12 Months Ended				12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Secured Debt [Member] properties	Dec. 31, 2011 Secured Debt [Member] properties	Dec. 31, 2012 Kilroy Realty, L.P. [Member]	Dec. 31, 2011 Kilroy Realty, L.P. [Member]	Dec. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] loans		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member]		Dec. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 4.27% Mortgage Payable due Feb 1, 2018 [Member]		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] 4.27% Mortgage Payable due Feb 1, 2018 [Member]		Jun. 30, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 4.48% Mortgage Payable due July 2027	Dec. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 4.48% Mortgage Payable due July 2027		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] 4.48% Mortgage Payable due July 2027		Jul. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 6.37% Mortgage Payable due April 2013 [Member]	Dec. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 6.37% Mortgage Payable due April 2013 [Member]		Jul. 24, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 6.37% Mortgage Payable due April 2013 [Member] building	Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] 6.37% Mortgage Payable due April 2013 [Member]		Dec. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 5.57% Mortgage Payable due August 1, 2012 [Member]		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] 5.57% Mortgage Payable due August 1, 2012 [Member]		Dec. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 6.51% Mortgage Payable due February 1, 2017 [Member]		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] 6.51% Mortgage Payable due February 1, 2017 [Member]		Jul. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 5.23% Mortgage Payable due January 2016 [Member] building	Dec. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 5.23% Mortgage Payable due January 2016 [Member]		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] 5.23% Mortgage Payable due January 2016 [Member]		Dec. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 5.57% Mortgage Note Payable due February 2016 [Member]		Oct. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 5.57% Mortgage Note Payable due February 2016 [Member] building	Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] 5.57% Mortgage Note Payable due February 2016 [Member]		Dec. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 5.09% Mortgage Payable due August 2015 [Member] [Member]		Jun. 30, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 5.09% Mortgage Payable due August 2015 [Member] [Member] building	Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] 5.09% Mortgage Payable due August 2015 [Member] [Member]		Dec. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 4.94% Mortgage Payable due April 15, 2015 [Member]		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] 4.94% Mortgage Payable due April 15, 2015 [Member]		Apr. 30, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] 4.94% Mortgage Payable due April 15, 2015 [Member]	Dec. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 4.95% Mortgage Payable due August 1, 2012 [Member]		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] 4.95% Mortgage Payable due August 1, 2012 [Member]		Dec. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] 7.15% Mortgage Payable due May 1, 2017 [Member]		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] 7.15% Mortgage Payable due May 1, 2017 [Member]		Dec. 31, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] Public Facility Bonds [Member]		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Secured Debt [Member] Public Facility Bonds [Member]		Jun. 30, 2012 Irvine, California [Member] Kilroy Realty, L.P. [Member] Secured Debt [Member] 4.48% Mortgage Payable due July 2027 building	Jun. 30, 2012 Los Angeles, California [Member] Kilroy Realty, L.P. [Member] Secured Debt [Member] 4.48% Mortgage Payable due July 2027 building
Secured Debt
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]			5.57%	[1],[2],[7]				5.09%	[1],[2],[8]				4.94%	[1],[2]				4.95%	[1],[5]			7.15%	[1]
Effective interest rate											4.27%	[1],[9]				4.48%	[1],[2],[3],[9]				3.55%	[1],[2],[4],[9]				5.57%	[1],[5],[9]			6.51%	[1],[9]				3.50%	[1],[2],[6],[9]			3.25%	[1],[2],[7],[9]				3.50%	[1],[2],[8],[9]				4.00%	[1],[2],[9]				4.95%	[1],[5],[9]			7.15%	[1],[9]
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]			Feb. 11, 2016	[2],[7]				Aug. 07, 2015	[2],[8]				Apr. 15, 2015	[2]				Aug. 01, 2012	[5]			May 01, 2017
Secured Debt	$ 561,096,000	$ 351,825,000			$ 561,096,000	$ 351,825,000	$ 561,096,000	[10]	$ 351,825,000	[10]	$ 135,000,000	[10]	$ 135,000,000	[10]		$ 97,000,000	[10],[2],[3]	$ 0	[10],[2],[3]		$ 83,116,000	[10],[2],[4]		$ 0	[10],[2],[4]	$ 0	[10],[5]	$ 71,517,000	[10],[5]	$ 68,615,000	[10]	$ 69,507,000	[10]		$ 56,302,000		$ 0	[10],[2],[6]	$ 43,016,000			$ 0	[10],[2],[7]	$ 35,379,000			$ 0	[10],[2],[8]	$ 28,941,000	[10],[2]	$ 30,191,000	[10],[2]		$ 0	[10],[5]	$ 29,754,000	[10],[5]	$ 11,210,000	[10]	$ 13,294,000	[10]	$ 2,517,000	[10],[11]	$ 2,562,000	[10],[11]
Debt amortization period															30 years					30 years														30 years
Period of interest only payments															3 years
Principal amount																							83,600,000											53,900,000							40,700,000					34,000,000							30,000,000
Debt premium							8,900,000																1,400,000											3,100,000							2,700,000					1,700,000							1,000,000
Number of buildings																							1											1							1					2																				1	2
Stated interest rate, minimum																																																														4.74%
Stated interest rate, maximum																																																														6.20%
Number of properties used as collateral for secured debbt			20	18
Net book value of properties pledged as collateral for secured debt			$ 1,010,200,000	$ 567,800,000
Number of secured loans with restrictions							9
Number of secured debt							10

[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 one office property located in Irvine, California and two office properties located in Los Angeles, California and requires monthly principal and interest payments based on a 30 year amortization period with an initial 3 years of interest only payments.

[4] In July 2012, in connection with the acquisition of one office building in Bellevue, Washington, we assumed a 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. The loan requires monthly principal and interest payments based on a 30 year amortization period. In January 2013, we repaid this loan prior to the stated maturity.

[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 a 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. The loan requires monthly principal and interest payments based on a 30 year amortization period.

[7] In October 2012, in connection with the acquisition of one office building in Los Angeles, California, we assumed a mortgage loan that is secured by the project. The assumed mortgage loan had a principal balance of $40.7 million at the acquisition date and was recorded at fair value at the date of acquisition resulting in an initial premium ofapproximately $2.7 million.

[8] 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.

[9] 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.

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

[11] 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.