Secured and Unsecured Debt of the Operating Partnership (Details 4) (USD $)	9 Months Ended			9 Months Ended			9 Months Ended	12 Months Ended		9 Months Ended		1 Months Ended	9 Months Ended		12 Months Ended			3 Months Ended		9 Months Ended			12 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 Kilroy Realty, L.P. [Member]	Sep. 30, 2011 Kilroy Realty, L.P. [Member]	Dec. 31, 2011 Kilroy Realty, L.P. [Member]	Sep. 30, 2012 Kilroy Realty, L.P. [Member] Revolving Credit Facility [Member]	Dec. 31, 2011 Kilroy Realty, L.P. [Member] Revolving Credit Facility [Member]	Dec. 31, 2011 Kilroy Realty, L.P. [Member] Line of Credit [Member]	Sep. 30, 2012 Kilroy Realty, L.P. [Member] 5.09% Mortgage Payable due August 7, 2015 [Member] Secured Debt [Member]		Jun. 30, 2012 Kilroy Realty, L.P. [Member] 4.48% Mortgage Payable due July 1, 2027 [Member] Secured Debt [Member] Years Y	Sep. 30, 2012 Kilroy Realty, L.P. [Member] 4.48% Mortgage Payable due July 1, 2027 [Member] Secured Debt [Member]		Dec. 31, 2011 Kilroy Realty, L.P. [Member] 3.25% Exchangeable Notes [Member] Exchangeable Notes [Member]		Sep. 30, 2012 Kilroy Realty, L.P. [Member] 3.25% Exchangeable Notes [Member] Exchangeable Notes [Member]	Sep. 30, 2012 Kilroy Realty, L.P. [Member] 4.25% Exchangeable Notes [Member] Exchangeable Notes [Member]		Sep. 30, 2012 Kilroy Realty, L.P. [Member] 4.25% Exchangeable Notes [Member] Exchangeable Notes [Member]		Sep. 30, 2011 Kilroy Realty, L.P. [Member] 4.25% Exchangeable Notes [Member] Exchangeable Notes [Member]	Dec. 31, 2011 Kilroy Realty, L.P. [Member] 4.25% Exchangeable Notes [Member] Exchangeable Notes [Member]	Sep. 30, 2012 Kilroy Realty, L.P. [Member] $150M Unsecured Term Loan Facility [Member] Unsecured Term Loan [Member]
Debt Instrument [Line Items]
Referenced Shares Of Common Stock Under Capped Call Options															1,121,201			4,800,796		4,800,796			4,800,796
Exchange price including effect of capped calls															$ 102.72			$ 42.81		$ 42.81			$ 42.81
Secured and Unsecured Debt of the Operating Partnership (Textuals) (Abstract)
Repayments of Secured Debt	$ 104,578,000	$ 5,295,000		$ 104,578,000	$ 5,295,000
Maturity Date										Aug. 07, 2015	[1],[2]		Jul. 01, 2027	[2],[3]	Apr. 15, 2012					Nov. 15, 2014			Nov. 15, 2014	Mar. 29, 2016
Effective interest rate										3.50%	[1],[2],[4],[5]		4.48%	[2],[3],[4],[5]	5.45%	[6]		7.13%	[6]	7.13%	[6]		7.13%
Amortization period of debt												30
Period of interest only payments												3
Exchange price, as adjusted																				$ 35.93	[7]	$ 35.93	$ 35.93
Fair value of the shares upon conversion if Exchangeable Notes were converted on specified date																		225,100,000		218,600,000		180,400,000
Principal amount															148,000,000		0	172,500,000		172,500,000			172,500,000
Amount by which fair value of the shares upon conversion exceeds principal amount of the Exchangeable Notes on the specified date																		52,600,000		46,100,000		7,900,000
Unsecured debt	1,130,814,000		980,569,000	1,130,814,000		980,569,000																		150,000,000
Basis spread on variable rate debt							1.75%		1.75%															1.75%
Debt Issuance Cost							8,300,000
Unsecured debt additional borrowing amount								$ 200,000,000																$ 100,000,000
Unsecured debt maturity extension							1 year	1 year																1 year
FMV cap rate							6.75%	7.50%

[1] 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.

[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] 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.

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

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

[7] 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.