Leases - Additional Information (Detail) $ in Millions	1 Months Ended				12 Months Ended
	Dec. 31, 2018 USD ($) ft² Space	Jun. 30, 2018 ft²	May 31, 2018 ft²	Aug. 31, 2017 ft² yr	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Leases [Line Items]
Rent expense | $					$ 8.1	$ 4.7	$ 3.2
Deferred rent | $	$ 9.3				$ 9.3	$ 6.0
ARE-San Francisco No. 19, LLC [Member] | Mission Bay Lease [Member]
Leases [Line Items]
Extended lease term start year				2020
Lease expiration year				2030
Lease space				134,356
Lease, option to extend, description					The term of the Mission Bay Lease commenced on September 1, 2017, and will expire January 31, 2030, subject to our right to extend the term of the lease for two consecutive five-year periods.
Number of consecutive 5 year terms to extend lease | yr				2
Lease extension term				5 years
Lease commencement date				Sep. 01, 2017
Lease expiration date				Jan. 31, 2030
Additional space required to be leased in the future				18,000
Kilroy Realty Finance Partnership, L.P [Member] | Third Street Lease [Member]
Leases [Line Items]
Lease space			135,936
Lease, option to extend, description					The lease term includes a right to extend the term for a consecutive five-year period.
Lease extension term			5 years
Lease expiration date			Jan. 31, 2030
Lease term start month and year			2018-06
Leased space delivered	67,105	1,726
Number of additional spaces delivered | Space	2
Remaining square feet of space expected delivery year			2019