Subsequent Events (Details) (USD $)	12 Months Ended				0 Months Ended		3 Months Ended	12 Months Ended								0 Months Ended
	Dec. 31, 2012 sqft		Dec. 31, 2011	Dec. 31, 2010	Jan. 15, 2013 Subsequent Event [Member]	Jan. 10, 2013 Subsequent Event [Member]	Mar. 31, 2013 Secured Debt [Member] 6.37% Mortgage Payable due April 2013 [Member] Subsequent Event [Member]	Dec. 31, 2012 Kilroy Realty, L.P. [Member]	Dec. 31, 2011 Kilroy Realty, L.P. [Member]	Dec. 31, 2010 Kilroy Realty, L.P. [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	Jan. 14, 2013 Kilroy Realty, L.P. [Member] Unsecured Debt [Member] Unsecured Senior Notes, 3 Point 8 Percent, Due 2023 [Member] Subsequent Event [Member]	Jan. 16, 2013 South Lake Union Submarket [Member] Subsequent Event [Member] sqft	Jan. 10, 2013 Restricted Stock Units (RSUs) [Member] Subsequent Event [Member]
Subsequent Event [Line Items]
Repayments of Secured Debt							$ 83,100,000
Principal amount													83,600,000	300,000,000
Stated interest rate											6.37%	[1],[2],[3]		3.80%
Subsequent Events (Textuals) [Abstract]
Aggregate dividends, distributions, and dividend equivalents paid to common stockholders and common unitholders	97,386,000		80,005,000	69,224,000	26,200,000			97,386,000	80,005,000	69,224,000
Rentable square feet	13,249,780	[4]													320,000
Business Combination, Consideration Transferred															170,000,000
Business Acquisition, Cost of Acquired Entity, Liabilities Incurred															83,900,000
Number of buildings													1
Granted (in shares)																157,744
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Fair Value																$ 7,700,000
Share Price						$ 48.88
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights																P5Y

[1] 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.

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

[3] 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.

[4] Includes square footage from our stabilized portfolio.