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Business Acquisition and Purchase of Technology - Additional Information (Detail) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Sep. 20, 2017
Sep. 30, 2017
Dec. 31, 2018
Dec. 31, 2017
Business Acquisition [Line Items]        
Cash consideration paid       $ 9,415
Goodwill     $ 14,950 $ 14,950
Deferred tax benefit from release of deferred tax asset valuation allowance $ 2,200      
Acquired Technology [Member]        
Business Acquisition [Line Items]        
Estimated useful life   2 years    
Intangible assets acquired   $ 400    
Motion AI, Inc. [Member]        
Business Acquisition [Line Items]        
Acquisition effective date     Sep. 20, 2017  
Business acquisition, description of acquired entity     On September 20, 2017, the Company acquired 100% of the equity interests of Motion AI, Inc., a Delaware technology corporation that allows users to scale one-to-one communications. The acquisition strengthens the Company’s position in the one-to-one communication space. Under the terms of the purchase agreement, the Company paid $9.0 million. The excess of the purchase consideration over the fair value of net tangible and identifiable intangible assets and liabilities acquired was recorded as goodwill and is primarily attributable to expanded market opportunities.  
Percentage of equity interests acquired 100.00%      
Cash consideration paid $ 9,000      
Business combination, assets acquired and liabilities assumed, tangible assets 32      
Goodwill 5,200      
Business combination, potential consideration not included in purchase price allocation 4,000      
Business combination potential consideration earned     $ 3,800  
Business combination remaining potential consideration     $ 200  
Remaining potential consideration recording period     2 years  
Motion AI, Inc. [Member] | Developed Technology [Member]        
Business Acquisition [Line Items]        
Estimated useful life     2 years  
Motion AI, Inc. [Member] | Acquired Technology [Member]        
Business Acquisition [Line Items]        
Estimated useful life     2 years  
Intangible assets acquired $ 6,000