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, 2017	Dec. 31, 2015	Dec. 31, 2016
Business Acquisition [Line Items]
Cash consideration paid			$ 9,415	$ 600
Goodwill			$ 14,950		$ 9,773
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
Potential consideration recording period, if earned	3 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		$ 6,000