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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 |  |  |  |

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