Organization and Significant Accounting Policies (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) message	Dec. 31, 2017 USD ($)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Monthly messages sent through company solutions | message	100,000,000
Reclassification to accumulated other comprehensive income	$ (1,508)	$ (1,088)
Accounting Standards Update 2014-09 | ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Reclassification from additional paid in capital	1,500	1,100
Reclassification to accumulated other comprehensive income	$ 1,500	$ 1,100