Secured and Unsecured Debt of the Operating Partnership - Exchangeable Senior Notes (Details) (USD $)	12 Months Ended						1 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended			1 Months Ended	12 Months Ended					12 Months Ended			12 Months Ended							12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010 Exchangeable Notes [Member] 3.25% Exchangeable Notes [Member]	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]	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		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	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, 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, 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, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member] Municipal Bonds [Member]	Dec. 31, 2012 Kilroy Realty, L.P. [Member] Exchangeable Notes [Member]	Dec. 31, 2011 Kilroy Realty, L.P. [Member] Exchangeable Notes [Member]	Dec. 31, 2010 Kilroy Realty, L.P. [Member] Exchangeable Notes [Member]	Dec. 31, 2011 Kilroy Realty, L.P. [Member] Exchangeable Notes [Member] 3.25% Exchangeable Notes [Member]		Dec. 31, 2012 Kilroy Realty, L.P. [Member] Exchangeable Notes [Member] 3.25% Exchangeable Notes [Member]	Dec. 31, 2012 Kilroy Realty, L.P. [Member] Exchangeable Notes [Member] 4.25% Exchangeable Notes [Member]		Dec. 31, 2011 Kilroy Realty, L.P. [Member] Exchangeable Notes [Member] 4.25% Exchangeable Notes [Member]
Debt Instrument [Line Items]
Number of Real Estate Properties													1	1					1			2
Balance and significant terms of the Exchangeable Notes outstanding
Principal amount																											$ 148,000,000		$ 0	$ 172,500,000	[1]	$ 172,500,000	[2]
Unamortized discount																											(924,000)		0	(8,556,000)		(12,684,000)
Net carrying amount of liability component	163,944,000	306,892,000		163,944,000	306,892,000																						147,076,000		0	163,944,000		159,816,000
Carrying amount of equity component																											33,675,000			19,835,000
Maturity date								Jul. 01, 2027	[3],[4]		Apr. 01, 2013	[3],[5]			Jan. 01, 2016	[3],[6]	Feb. 11, 2016	[3],[7]		Aug. 07, 2015	[3],[8]						Apr. 15, 2012
Stated interest rate			3.25%					4.48%	[3],[4],[9]		6.37%	[3],[5],[9]			5.23%	[3],[6],[9]	5.57%	[3],[7],[9]		5.09%	[3],[8],[9]			4.25%			3.25%	[10]	3.25%	4.25%	[10]
Effective interest rate								4.48%	[11],[3],[4],[9]		3.55%	[11],[3],[5],[9]			3.50%	[11],[3],[6],[9]	3.25%	[11],[3],[7],[9]		3.50%	[11],[3],[8],[9]						5.45%	[12]		7.13%	[12]
Exchange rate per $1,000 principal value of the Exchangeable Notes, as adjusted																														27.8307	[13]
Exchange price, as adjusted																														$ 35.93	[13]
Number of shares on which aggregate consideration to be delivered upon conversion is determined																														4,800,796	[13]
Average trading price of the company's stock	$ 45.72	$ 37.27
Fair value of the shares upon conversion if Exchangeable Notes were converted on specified date																														221,200,000	[1]	179,100,000	[2]
Excess amount of if-converted value over the principal amount																														48,700,000	[1]	6,600,000	[2]
Interest expense for the Exchangeable Notes
Contractual interest payments																								8,721,000	12,141,000	14,565,000
Amortization of discount																								5,052,000	6,928,000	7,965,000
Interest expense attributable to the Exchangeable Notes																								13,773,000	19,069,000	22,530,000
Capped call transactions
Referenced shares of common stock																											1,121,201			4,800,796	[14]
Exchange price including effect of capped calls																											$ 102.72			$ 42.81	[14]
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
Debt premium						$ 8,900,000							$ 1,400,000	$ 3,100,000					$ 2,700,000			$ 1,700,000
Stated interest rate, minimum																							4.74%
Stated interest rate, maximum																							6.20%

[1] Although the 4.25% Exchangeable Notes were exchangeable during the three months ended December 31, 2012, no 4.25% Exchangeable Notes were exchanged during the period. The 4.25% Exchangeable Notes were not exchangeable during the remainder of the year.

[2] The 4.25% Exchangeable Notes were not exchangeable during the year ended December 31, 2011.

[3] 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.

[4] 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.

[5] 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.

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

[10] Interest on the 4.25% Exchangeable Notes is payable semi-annually in arrears on May 15th and November 15th of each year.

[11] 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.

[12] The rate at which we record interest expense for financial reporting purposes, which reflects the amortization of the discounts on the Exchangeable Notes (see Note 2). This rate represents our conventional debt borrowing rate at the date of issuance.

[13] The exchange rate, exchange price, and the number of shares to be delivered upon conversion are subject to adjustment under certain circumstances including increases in our common dividends.

[14] The capped calls mitigate the dilutive impact to us of the potential exchange of all of the 4.25% Exchangeable Notes into shares of common stock.