Secured and Unsecured Debt of the Operating Partnership (Details 1) (USD $)	3 Months Ended			12 Months Ended
	Mar. 31, 2013 D		Mar. 31, 2012	Dec. 31, 2012
Balance and significant terms of the Exchangeable Notes outstanding
Net carrying amount of liability component	$ 165,022,000			$ 163,944,000
Per Share Average Trading Price Of Companys Common Stock On Stock Exchange Abstract
Average trading price of the Companys stock	$ 51.14		$ 42.86
Kilroy Realty, L.P. [Member]
Balance and significant terms of the Exchangeable Notes outstanding
Net carrying amount of liability component	165,022,000			163,944,000
Kilroy Realty, L.P. [Member] | Revolving Credit Facility [Member]
Capped call transactions
Line of Credit Facility, Commitment Fee Percentage	0.30%	[1]		0.30%
Line of Credit Facility, Expiration Date	Apr. 03, 2017	[2]		Apr. 03, 2017
Debt Instrument, Basis Spread on Variable Rate	1.45%			1.45%
Unsecured debt additional borrowing amount	200,000,000			200,000,000
Kilroy Realty, L.P. [Member] | Secured Debt [Member] | 6.05% Mortgage Payable Due June 1, 2019 [Member]
Balance and significant terms of the Exchangeable Notes outstanding
Maturity Date	Jun. 01, 2019	[3],[4]
Stated coupon rate	6.05%	[3],[4],[5]			[3],[4],[5]
Effective interest rate	3.50%	[3],[4],[5],[6]			[3],[4],[5],[6]
Kilroy Realty, L.P. [Member] | Exchangeable Notes [Member]
Interest Expense for the Exchangeable Notes
Contractual interest payments	1,833,000	[7]	3,035,000
Amortization of discount	1,078,000	[7]	1,797,000
Interest expense attributable to the Exchangeable Notes	2,911,000	[7]	4,832,000
Kilroy Realty, L.P. [Member] | Exchangeable Notes [Member] | 4.25% Exchangeable Notes [Member]
Balance and significant terms of the Exchangeable Notes outstanding
Principal amount	172,500,000			172,500,000
Unamortized discount	(7,478,000)			(8,556,000)
Net carrying amount of liability component	165,022,000			163,944,000
Carrying amount of equity component	19,835,000			19,835,000
Debt Instrument, Issuance Date	Nov. 15, 2009			Nov. 15, 2009
Maturity Date	Nov. 15, 2014			Nov. 15, 2014
Stated coupon rate	4.25%	[8]		4.25%	[8]
Effective interest rate	7.13%	[9]		7.13%
Exchange rate per $1,000 principal value of the Exchangeable Notes, as adjusted	27.8307	[10]		27.8307
Exchange price, as adjusted	$ 35.93	[10]		$ 35.93
Number of Shares on which aggregate consideration to be delivered upon conversion is determined	4,800,796	[10]		4,800,796
Minimum percentage of exchange price	130.00%
Minimum number of trading days closing per share price is higher than exchange price	20
Per Share Average Trading Price Of Companys Common Stock On Stock Exchange Abstract
Fair Value Of Shares Upon Conversion If Exchangeable Notes Were Converted On Specified Date	247,300,000		208,700,000
Debt Instrument, Convertible, If-converted Value in Excess of Principal	$ 74,800,000		$ 36,200,000
Capped call transactions
Referenced shares of common stock	4,800,796			4,800,796
Exchange price including effect of capped calls	$ 42.81			$ 42.81

[1] The facility fee is paid on a quarterly basis and is calculated based on the total borrowing capacity. In addition to the facility fee, from 2010 to 2012 we incurred debt origination and legal costs totaling approximately $10.2 million that are currently being amortized through the maturity date of the revolving credit facility.

[2] Under the terms of the revolving credit facility, we may exercise an option to extend the maturity date by one year.

[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 January 2013, 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 had a principal balance of $83.9 million at the acquisition date and was recorded at fair value on the date of the acquisition resulting in a premium of approximately $11.6 million. The loan requires monthly principal and interest payments based on a 6.4 year amortization period.

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

[6] 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.

[7] The Company repaid the 3.25% Exchangeable Notes in April 2012. Interest payments and discount amortization for the three months ended March 31, 2013 are solely attributable to the 4.25% Exchangeable Notes.

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

[9] The rate at which we record interest expense for financial reporting purposes, which reflects the amortization of the discounts on the 4.25% Exchangeable Notes. This rate represents our conventional debt borrowing rate at the date of issuance.

[10] 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.