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Leases - Additional Information (Detail)
 1 Months Ended 12 Months Ended
Aug. 31, 2017
ft²
yr
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Leases [Line Items]        
Rent expense   $ 4,700,000 $ 3,200,000 $ 3,200,000
Deferred rent   6,000,000 6,600,000  
Repayment of capital lease   5,131,000 $ 5,945,000 $ 5,187,000
Capital lease balance   $ 0    
ARE-San Francisco No. 19, LLC [Member]        
Leases [Line Items]        
Extended lease term start year 2020      
Lease expiration year 2030      
Lease space | ft² 129,732      
Lease description option to extend   The term of the 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 | ft² 23,000