Basis of Presentation and Significant Accounting Policies (Details) (USD $)	3 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 VIEtransactions VIE building		Dec. 31, 2012 NumberofTradingDays segment VIE building		Dec. 31, 2011 segment VIE		Dec. 31, 2010	Dec. 31, 2012 Undeveloped Land [Member]	Dec. 31, 2011 Series A Cummulative Redeemable Preferred Unit [Member] Kilroy Realty, L.P. [Member]	Dec. 31, 2012 Series A Cummulative Redeemable Preferred Unit [Member] Kilroy Realty, L.P. [Member]	Dec. 31, 2010 Series A Cummulative Redeemable Preferred Unit [Member] Kilroy Realty, L.P. [Member]	Dec. 31, 2011 Series E Cumulative Redeemable Preferred Unit [Member] Kilroy Realty, L.P. [Member]	Dec. 31, 2012 Series E Cumulative Redeemable Preferred Unit [Member] Kilroy Realty, L.P. [Member]	Dec. 31, 2011 Series F Cumulative Redeemable Preferred Unit [Member] Kilroy Realty, L.P. [Member]	Dec. 31, 2012 Series F Cumulative Redeemable Preferred Unit [Member] Kilroy Realty, L.P. [Member]	Dec. 31, 2012 Series G Cumulative Redeemable Preferred Unit [Member] Kilroy Realty, L.P. [Member]	Mar. 31, 2012 Series G Cumulative Redeemable Preferred Unit [Member] Kilroy Realty, L.P. [Member]	Dec. 31, 2011 Series G Cumulative Redeemable Preferred Unit [Member] Kilroy Realty, L.P. [Member]	Dec. 31, 2012 Series H Cumulative Redeemable Preferred Unit [Member] Kilroy Realty, L.P. [Member]	Aug. 31, 2012 Series H Cumulative Redeemable Preferred Unit [Member] Kilroy Realty, L.P. [Member]	Dec. 31, 2011 Series H Cumulative Redeemable Preferred Unit [Member] Kilroy Realty, L.P. [Member]	Dec. 31, 2012 Largest Tenants [Member] NumberofTenants	Dec. 31, 2012 Secured Debt [Member]	Dec. 31, 2011 Secured Debt [Member]	Dec. 31, 2012 Unsecured Debt [Member]	Dec. 31, 2011 Unsecured Debt [Member]	Dec. 31, 2012 Office Properties Acquisitions [Member] Variable Interest Entity, Primary Beneficiary, Aggregated Disclosure [Member] building	Dec. 31, 2012 Development and Redevelopment Project Acquisitions [Member] project	Dec. 31, 2012 Development and Redevelopment Project Acquisitions [Member] Variable Interest Entity, Primary Beneficiary, Aggregated Disclosure [Member] property	Dec. 31, 2012 Real Estate Investment [Member]	Dec. 31, 2011 Real Estate Investment [Member]	Dec. 31, 2012 Minimum [Member] Building and Building Improvements [Member]	Dec. 31, 2012 Minimum [Member] Tenant Improvements [Member]	Dec. 31, 2012 Maximum [Member] Building and Building Improvements [Member]	Dec. 31, 2012 Maximum [Member] Tenant Improvements [Member]	Dec. 31, 2011 Assets Held-for-sale [Member] building	Dec. 31, 2012 Lease Agreements [Member] Minimum [Member]	Dec. 31, 2012 Lease Agreements [Member] Maximum [Member]
Basis of presentation and significant accounting policies [Line Items]
Business Acquisition, Acquisition Related Costs Capitalized								$ 700,000
Number of consolidated variable interest entities	2		2		1
Number of days to complete Section 1031 Exchange			180
VIE Asset Amount	337,000,000		337,000,000		108,500,000																									319,770,000	101,352,000
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities	111,100,000		111,100,000		7,300,000
Secured debt	221,032,000	[1]	221,032,000	[1]	30,997,000	[2]
Number of buildings																											1	7	1							2
Number of acquisition transactions	2
Depreciation and amortization of buildings and improvements			125,900,000		106,000,000		86,300,000
Property, Plant and Equipment, Useful Life																																25 years	1 year	40 years	20 years
Finite-Lived Intangible Asset, Useful Life																																						20 years
Leases, Estimated Useful Lives																																					1 year
Amortization of Financing Costs			16,600,000		16,800,000
Unamortized discount (premium), Net																							(7,200,000)	(800,000)	2,100,000	2,400,000
Preferred stock issued (in shares)										1,510,000	1,510,000
Preferred Units, Issued												1,610,000		3,450,000		4,000,000	4,000,000		4,000,000	4,000,000
Preferred stock (in shares)									1,510,000		1,510,000
Preferred units outstanding												1,610,000		3,450,000		4,000,000			4,000,000
Preferred stock, dividend rate									7.45%			7.80%		7.50%		6.875%			6.375%
Percentage of General partnership interest owned by wholly-owned subsidiary of the Company	1.00%		1.00%
REIT annual taxable income distribution requirement percentage			90.00%		90.00%
Company annual taxable income distribution			100.00%
Income tax liability relating to taxable REIT subisidiary			$ 60,000,000		$ 30,000,000		$ 30,000,000
Concentration Risk, Geographic			All of our properties and all of our business is currently conducted in the state of California with the exception of the ownership and operation of ten office properties located in the state of Washington. The ability of the tenants to honor the terms of their leases is dependent upon the economic, regulatory, and social factors affecting the communities in which the tenants operate.
Concentration Risk, Customer			As of December 31, 2012, our 15 largest tenants represented approximately 34.1% of total annualized base rental revenues.
Number Of Properties Of Company In Another State	10		10
Number Of Tenants																						15,000
Concentration Risk of Significant Tenants, Percentage			34.10%
Concentration Risk, Credit Risk, Uninsured Deposits			We have deposited cash with financial institutions that is insured by the Federal Deposit Insurance Corporation (“FDIC”) up to $250,000 per institution. As of December 31, 2012 and 2011, we had cash accounts in excess of FDIC insured limits.
Number of Reportable Segments			1		1
Number of Non-Reportable Segments					1

[1] Represents the fair value of the mortgage loans assumed, which includes an aggregate unamortized premium balance of approximately $8.9 million at the dates of acquisition (see Note 7).

[2] Reflects the purchase price net of assumed secured debt and other lease-related obligations.