Summary of Significant Accounting Policies - Reconciliation of Shares Used to Determine Net Income per Share (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Earnings Per Share Disclosure [Line Items]
Net (loss) income	$ (30,616)	$ 8,525	$ (33,672)	$ 14,259	$ 11,157
Weighted average number of shares - basic	42,403,140	41,567,848	42,403,084	41,537,894
Shares underlying options	0	1,982,378	0	1,953,020
Weighted average number of shares outstanding - diluted	42,403,140	43,550,226	42,403,084	43,490,914