Unsecured Revolving Credit Facility (Details 3) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2015 USD ($) building	Dec. 31, 2014 USD ($)
Debt Instrument [Line Items]
Number of Real Estate Properties | building		10
Unsecured debt		$ 2,181,382	$ 1,783,121
Terms of the Credit Facility
Unsecured line of credit		0	140,000
Kilroy Realty, L.P. [Member]
Debt Instrument [Line Items]
Unsecured debt		2,181,382	1,783,121
Terms of the Credit Facility
Unsecured line of credit		0	140,000
Kilroy Realty, L.P. [Member] | Revolving Credit Facility [Member]
Terms of the Credit Facility
Unsecured line of credit		0	140,000
Remaining borrowing capacity		600,000	460,000
Total borrowing capacity	[1]	$ 600,000	$ 600,000
Line of Credit Facility, Interest Rate at Period End	[2]	0.00%	1.41%
Line of Credit Facility, Commitment Fee Percentage	[3]	0.20%	0.25%
Line of Credit Facility, Expiration Date		Jul. 01, 2019
Debt Instrument, Contingent Additional Borrowings		$ 311,000
Debt Instrument, Basis Spread on Variable Rate		1.05%	1.25%
Debt Issuance Cost		$ 4,900
Term Loan Facility [Member] | Kilroy Realty, L.P. [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Unsecured debt		$ 150,000	$ 150,000
Terms of the Credit Facility
Debt Instrument, Basis Spread on Variable Rate	[4]	1.35%	1.56%
Maturity Date		Jul. 01, 2019
Term Loan Facility [Member] | Kilroy Realty, L.P. [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Unsecured debt		$ 39,000
London Interbank Offered Rate (LIBOR) [Member] | Term Loan Facility [Member] | Kilroy Realty, L.P. [Member] | Line of Credit [Member]
Terms of the Credit Facility
Debt Instrument, Basis Spread on Variable Rate		1.15%	1.40%

[1] We may elect to borrow, subject to bank approval and obtaining commitments for any additional borrowing capacity, up to an additional $311.0 million under an accordion feature under the terms of the unsecured revolving credit facility and term loan facility.

[2] Our unsecured revolving credit facility interest rate was calculated based on an annual rate of LIBOR plus 1.050% and LIBOR plus 1.250% as of September 30, 2015 and December 31, 2014, respectively.

[3] Our facility fee is paid on a quarterly basis and is calculated based on the total borrowing capacity. In addition to the facility fee, we incurred debt origination and legal costs. As of September 30, 2015, $4.9 million of deferred financing costs remains to be amortized through the maturity date of our unsecured revolving credit facility.

[4] Our unsecured term loan facility interest rate was calculated based on an annual rate of LIBOR plus 1.150% and LIBOR plus 1.400% as of September 30, 2015 and December 31, 2014, respectively.