Secured and Unsecured Debt of the Operating Partnership - Unsecured Senior Notes Table (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Nov. 30, 2018
Debt Instrument [Line Items]
Unsecured debt, net	$ 3,049,185		$ 2,552,070
Unsecured debt [Member]
Debt Instrument [Line Items]
Unamortized discount (premium), net	6,500		6,600
Unamortized debt issuance expense	18,700		15,400
Kilroy Realty L.P. [Member]
Debt Instrument [Line Items]
Unamortized debt issuance expense	20,300
Long-term debt, gross	3,579,502
Unsecured debt, net	3,049,185		2,552,070
Kilroy Realty L.P. [Member] | Unsecured debt [Member]
Debt Instrument [Line Items]
Unsecured debt, net	$ 2,899,839		2,402,979
Kilroy Realty L.P. [Member] | Unsecured debt [Member] | 3.050% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	3.05%	3.05%
Effective interest rate	3.064%
Long-term debt, gross	$ 500,000		0
Unamortized discount and deferred financing costs	(5,998)		0
Unsecured debt, net	$ 494,002		$ 0
Kilroy Realty L.P. [Member] | Unsecured debt [Member] | 4.750% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Stated interest rate			4.75%	4.75%
Effective interest rate	4.80%
Long-term debt, gross	$ 400,000		$ 400,000
Unamortized discount and deferred financing costs	(4,446)		(4,960)
Unsecured debt, net	$ 395,554		395,040
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	(1,186)		(1,375)
Unsecured debt, net	$ 198,814		198,625
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	(290)		(342)
Unsecured debt, net	$ 49,710		49,658
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	(2,907)		(3,493)
Unsecured debt, net	$ 422,093		421,507
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	(390)		(432)
Unsecured debt, net	$ 74,610		74,568
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	(825)		(941)
Unsecured debt, net	$ 174,175		174,059
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	(3,185)		(3,738)
Unsecured debt, net	$ 396,815		396,262
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	(5,100)		(5,632)
Unsecured debt, net	$ 394,900		394,368
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	(834)		(1,108)
Unsecured debt, net	$ 299,166		$ 298,892