Summary of Significant Accounting Policies - Reconciliation of Shares Used to Determine Net Income per Share (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Earnings Per Share Disclosure [Line Items]
Net (loss) income	$ (13,880)	$ 2,104	$ (47,552)	$ 16,362	$ 11,157
Weighted average number of shares - basic	42,530,476	41,869,528	42,445,926	41,648,439
Shares underlying options	0	1,594,454	0	1,524,988
Weighted average number of shares outstanding – diluted	42,530,476	43,463,982	42,445,926	43,173,427