Leases - Additional Information (Detail) $ in Thousands	1 Months Ended			12 Months Ended
	Jun. 30, 2018 ft²	May 31, 2018 ft²	Aug. 31, 2017 ft² extension_period	Dec. 31, 2019 USD ($) ft²	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2019	Aug. 31, 2019 ft²
Leases [Line Items]
Operating lease right-of-use assets				$ 134,177	$ 0
Operating lease, liability				155,246
Right-of-use assets recognized in exchange for operating lease liabilities				57,691	0	$ 0
Operating lease cost				14,697	7,972	4,515
Operating lease, payments				$ 8,400
Weighted average remaining lease term				10 years 1 month 6 days
Weighted average discount rate, percent							5.90%
Deferred rent					9,300
Variable lease cost				$ 6,408	4,497	3,077
Total lease costs				$ 21,105	$ 12,469	$ 7,592
ARE-San Francisco No. 19, LLC | Mission Bay Lease
Leases [Line Items]
Lease space | ft²			148,263
Number of consecutive 5 year terms to extend lease | extension_period			2
Lease extension term			5 years
Leased space delivered | ft²				13,907
Operating lease right-of-use assets				$ 6,700
Operating lease, liability				6,700
Additional space required to be leased in the future | ft²			4,940
Kilroy Realty Finance Partnership, L.P | Third Street Lease
Leases [Line Items]
Lease space | ft²		135,936						67,105
Leased space delivered | ft²	1,726	67,105
Right-of-use assets recognized in exchange for operating lease liabilities				$ 51,000