Common Stock - Reserved Shares of Common Stock (Detail) - shares shares in Thousands	Sep. 30, 2017	Dec. 31, 2016	Feb. 28, 2013
Class Of Stock [Line Items]
Reserved shares of Common Stock	2,261	2,040
Options or RSUs Available for Future Grant [Member]
Class Of Stock [Line Items]
Reserved shares of Common Stock	1,240	839
Options Outstanding [Member]
Class Of Stock [Line Items]
Reserved shares of Common Stock	438	536
Shares to be Issued in Connection with Acquisition of SocialMoov [Member]
Class Of Stock [Line Items]
Reserved shares of Common Stock	0	66
RSUs [Member]
Class Of Stock [Line Items]
Reserved shares of Common Stock	373	429
Shares Available for Future Issuance under Employee Stock Purchase Plan [Member]
Class Of Stock [Line Items]
Reserved shares of Common Stock	210	170	143