Secured and Unsecured Debt of the Operating Partnership - Unsecured Senior Notes Table (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Aug. 31, 2020	Dec. 31, 2019	Sep. 30, 2019
Debt Instrument [Line Items]
Unsecured debt, net	$ 3,670,099		$ 3,049,185
Unsecured debt [Member]
Debt Instrument [Line Items]
Unamortized discount (premium), net	(8,300)		(6,500)
Unamortized debt issuance expense	21,600		18,700
Kilroy Realty L.P. [Member]
Debt Instrument [Line Items]
Unamortized debt issuance expense	22,400
Long-term debt, gross	3,954,365
Unsecured debt, net	3,670,099		3,049,185
Kilroy Realty L.P. [Member] | Unsecured debt [Member]
Debt Instrument [Line Items]
Unsecured debt, net	$ 3,670,099		2,899,839
Kilroy Realty L.P. [Member] | Unsecured debt [Member] | 2.500% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Unamortized discount (premium), net		$ (2,700)
Stated interest rate	2.50%
Effective interest rate	2.56%
Long-term debt, gross	$ 425,000		0
Unamortized discount and deferred financing costs	(6,332)		0
Unsecured debt, net	$ 418,668		0
Kilroy Realty L.P. [Member] | Unsecured debt [Member] | 4.270% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	4.27%
Effective interest rate	4.27%
Long-term debt, gross	$ 350,000		0
Unamortized discount and deferred financing costs	(1,825)		0
Unsecured debt, net	$ 348,175		0
Kilroy Realty L.P. [Member] | Unsecured debt [Member] | 3.050% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Unamortized discount (premium), net				$ (600)
Stated interest rate	3.05%
Effective interest rate	3.064%
Long-term debt, gross	$ 500,000		500,000
Unamortized discount and deferred financing costs	(5,399)		(5,998)
Unsecured debt, net	$ 494,601		494,002
Kilroy Realty L.P. [Member] | Unsecured debt [Member] | 4.750% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	4.75%
Effective interest rate	4.80%
Long-term debt, gross	$ 400,000		400,000
Unamortized discount and deferred financing costs	(3,952)		(4,446)
Unsecured debt, net	$ 396,048		395,554
Kilroy Realty L.P. [Member] | Unsecured debt [Member] | 4.350% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	4.35%
Effective interest rate	4.35%
Long-term debt, gross	$ 200,000		200,000
Unamortized discount and deferred financing costs	(1,012)		(1,186)
Unsecured debt, net	$ 198,988		198,814
Kilroy Realty L.P. [Member] | Unsecured debt [Member] | 4.300% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	4.30%
Effective interest rate	4.30%
Long-term debt, gross	$ 50,000		50,000
Unamortized discount and deferred financing costs	(246)		(290)
Unsecured debt, net	$ 49,754		49,710
Kilroy Realty L.P. [Member] | Unsecured debt [Member] | 3.450% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	3.45%
Effective interest rate	3.47%
Long-term debt, gross	$ 425,000		425,000
Unamortized discount and deferred financing costs	(2,321)		(2,907)
Unsecured debt, net	$ 422,679		422,093
Kilroy Realty L.P. [Member] | Unsecured debt [Member] | 3.450% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	3.45%
Effective interest rate	3.45%
Long-term debt, gross	$ 75,000		75,000
Unamortized discount and deferred financing costs	(347)		(390)
Unsecured debt, net	$ 74,653		74,610
Kilroy Realty L.P. [Member] | Unsecured debt [Member] | 3.350% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	3.35%
Effective interest rate	3.35%
Long-term debt, gross	$ 175,000		175,000
Unamortized discount and deferred financing costs	(709)		(825)
Unsecured debt, net	$ 174,291		174,175
Kilroy Realty L.P. [Member] | Unsecured debt [Member] | 4.375% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	4.375%
Effective interest rate	4.444%
Long-term debt, gross	$ 400,000		400,000
Unamortized discount and deferred financing costs	(2,631)		(3,185)
Unsecured debt, net	$ 397,369		396,815
Kilroy Realty L.P. [Member] | Unsecured debt [Member] | 4.250% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	4.25%
Effective interest rate	4.35%
Long-term debt, gross	$ 400,000		400,000
Unamortized discount and deferred financing costs	(4,567)		(5,100)
Unsecured debt, net	$ 395,433		394,900
Kilroy Realty L.P. [Member] | Unsecured debt [Member] | 3.800% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	3.80%
Effective interest rate	3.80%
Long-term debt, gross	$ 300,000		300,000
Unamortized discount and deferred financing costs	(560)		(834)
Unsecured debt, net	$ 299,440		$ 299,166