Capitalized Software Development Costs - Summary of Capitalized Software Development Costs (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Capitalized Computer Software Net [Line Items]
Gross carrying amount	$ 43,915	$ 36,899
Accumulated amortization	(31,181)	(26,894)
Net carrying amount	12,734	10,005
Capitalized software development costs [Member]
Capitalized Computer Software Net [Line Items]
Gross carrying amount	$ 43,915	$ 36,899
Amortization period	3 years	3 years
Accumulated amortization	$ (31,181)	$ (26,894)
Net carrying amount	$ 12,734	$ 10,005