Acquisitions (Details Textuals) (USD $)	9 Months Ended																9 Months Ended		0 Months Ended						9 Months Ended
	Sep. 30, 2012 NumberofBuildings		Sep. 30, 2012 In-Place Leases [Member]	Sep. 30, 2012 Above-Market Leases [Member]	Sep. 30, 2012 Deferred Leasing Costs [Member]	Sep. 30, 2012 Below Market Ground Lease Obligation [Member]	Sep. 30, 2012 Below Market Leases [Member]	Sep. 30, 2012 Above Market Ground Lease Member	Sep. 30, 2012 Office Properties Acquisitions [Member] NumberofBuildings	Sep. 30, 2012 Material Acquisitions [Member]	Sep. 30, 2012 701 and 801 N. 34th St., Seattle, WA [Member] NumberofBuildings		Sep. 30, 2012 Development and Redevelopment Project Acquisitions [Member] project	Sep. 30, 2012 690 E. Middlefield, Mountain View, CA [Member]	May 09, 2012 690 E. Middlefield, Mountain View, CA [Member]	Sep. 28, 2012 6121 Sunset Boulevard, Los Angeles, CA [Member]	Sep. 30, 2012 Maximum [Member]	Sep. 30, 2012 Kilroy Realty, L.P. [Member] Material Acquisitions [Member]	Jul. 31, 2012 Kilroy Realty, L.P. [Member] 6255 Sunset Boulevard, Los Angeles, CA [Member]	Sep. 30, 2012 Kilroy Realty, L.P. [Member] Secured Debt [Member]	Sep. 30, 2012 5.09% Mortgage Payable due August 7, 2015 [Member] Kilroy Realty, L.P. [Member] Secured Debt [Member] NumberofBuildings	Oct. 05, 2012 Six Point Thirty Seven Percent Mortgage Payable due April First Two Thousand Thirteen [Member] Kilroy Realty, L.P. [Member] Secured Debt [Member]	Aug. 31, 2012 Five Point Twenty Three Percent Mortgage Payable due January First Two Thousand Sixteen [Member] Kilroy Realty, L.P. [Member] Secured Debt [Member] NumberofBuildings	Jul. 31, 2012 Five Point Twenty Three Percent Mortgage Payable due January First Two Thousand Sixteen [Member] Kilroy Realty, L.P. [Member] Secured Debt [Member]	Sep. 30, 2012 Ten Year Ground Lease Extension Option [Member] Extensions	Sep. 30, 2012 Forty-Five Year Ground Lease Extension Option [Member] Extensions	Sep. 30, 2012 Reportable Segment [Member] NumberofBuildings	Sep. 30, 2012 Reportable Segment [Member] 6255 Sunset Boulevard, Los Angeles, CA [Member] NumberofBuildings		Jul. 31, 2012 Reportable Segment [Member] Kilroy Realty, L.P. [Member] 6255 Sunset Boulevard, Los Angeles, CA [Member]
Aquisitions (Textuals) [Abstract]
Number of buildings	12								12		2	[1]	3								2		1				111	1	[2]
Cash assumed	$ 3,973,000										$ 4,000,000
Other assets assumed											200,000		1,300,000
Current liabilities assumed	6,348,000										600,000		7,227,000
Principal amount of secured debt																					34,000,000	83,600,000		53,900,000
Debt premium																				6,200,000	1,700,000	1,400,000		3,100,000
Units issued in acquisition																			118,372
Share Price																			$ 47.34
Liabilities assumed in conection with acquisition																														4,700,000
Purchase price for major acquisitions as percentage of purchase price of all acquisitions										61.00%								61.00%
Purchase price of aach acquisition as percentage of Companys assets																	5.00%
Purchase price of all acquisitions as percentage of Company assets																	10.00%
Number of extension options																									3	1
Ten Year Ground Lease Extension Option	10 years
Forty-Five Year Ground Lease Extension Option	45 years
Deferred leasing costs and acquisition-related intangible assets	55,729,000	[3]	41,100,000	1,400,000	12,800,000	500,000
Weighted average amortization period in years for deferred leasing and acquisition-related intangible assets			4 years 7 months 6 days	3 years 9 months 12 days	3 years 5 months 6 days	59 years 7 months 6 days
Acquisition-related intangible liabilities	34,570,000	[4]					33,500,000	1,100,000
Weighted average amortization period in years for acquisition-related liabilities							6 years 6 months 12 days	29 years 7 months 6 days
Business Acquisition, Percentage of Voting Interests Acquired															100.00%
Cash paid for purchase														74,500,000
Liabilities accrued for purchase																1,100,000
Leasing commissions assumed as part of purchase														$ 9,500,000

[1] We acquired these properties through the acquisition of the ownership interest of the bankruptcy remote LLC that owns the properties. In connection with this acquisition we also acquired cash of approximately $4.0 million, other assets of approximately $0.2 million and we assumed current liabilities of approximately $0.6 million and secured debt with an outstanding principal balance of $34.0 million and a premium of $1.7 million as a result of recording the debt at fair value at the acquisition date (see Note 5.)

[2] As part of the consideration for this transaction, we issued 118,372 common units of the Operating Partnership "OP units" valued at $47.34, which was the Company's closing stock price on the NYSE on the acquisition date. In connection with this acquisition we also assumed secured debt with an outstanding principal balance of $53.9 million and a premium of $3.1 million as a result of recording this debt at fair value on the acquisition date (see Note 5). We also assumed $4.7 million of accrued liabilities in connection with this acquisition which are not included in the purchase price above.

[3] Represents in-place leases (approximately $41.1 million with a weighted average amortization period of 4.6 years), above-market leases (approximately $1.4 million with a weighted average amortization period of 3.8 years), leasing commissions (approximately $12.8 million with a weighted average amortization period of 3.4 years), and a below-market ground lease obligation (approximately $0.5 million with a weighted average amortization period of 59.6 years).

[4] Represents below-market leases (approximately $33.5 million with a weighted average amortization period of 6.5 years) and an above-market ground lease obligation (approximately $1.1 million with a weighted average amortization period of 29.6 years).