Secured and Unsecured Debt of the Operating Partnership - Unsecured Revolving Credit Facility and Term Loan Facility (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Terms of the Credit Facility
Outstanding borrowings	$ 185,000,000	$ 45,000,000
Kilroy Realty L.P. [Member]
Terms of the Credit Facility
Outstanding borrowings	185,000,000	45,000,000
Unamortized deferred financing costs	16,700,000
Kilroy Realty L.P. [Member] | Revolving credit facility [Member]
Terms of the Credit Facility
Outstanding borrowings	185,000,000	45,000,000
Remaining borrowing capacity	565,000,000	705,000,000
Total borrowing capacity	$ 750,000,000	$ 750,000,000
Interest rate (percent)	3.50%	3.48%
Facility fee-annual rate (percent)	0.20%	0.20%
Contingent additional borrowings	$ 600,000,000
Unamortized deferred financing costs	$ 4,400,000	$ 4,700,000
Kilroy Realty L.P. [Member] | Revolving credit facility [Member] | London Interbank Offered Rate (LIBOR) [Member]
Terms of the Credit Facility
Variable rate (percent)	1.00%	1.00%
Kilroy Realty L.P. [Member] | Line of credit [Member] | $150 Million Term Loan Facility [Member]
Terms of the Credit Facility
Outstanding borrowings	$ 150,000,000	$ 150,000,000	$ 150,000,000
Remaining borrowing capacity	$ 0	0
Total borrowing capacity		$ 150,000,000
Interest rate (percent)	3.60%	3.49%
Undawn facility fee-annual rate (percent)	0.20%	0.20%
Unamortized deferred financing costs	$ 800,000	$ 900,000
Kilroy Realty L.P. [Member] | Line of credit [Member] | $150 Million Term Loan Facility [Member] | London Interbank Offered Rate (LIBOR) [Member]
Terms of the Credit Facility
Variable rate (percent)	1.10%	1.10%