Equity - Narrative (Detail) - USD ($)			1 Months Ended	3 Months Ended		9 Months Ended
	May 25, 2017	May 24, 2017	Aug. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense (gain)				$ 1,100,000	$ 2,800,000	$ 5,000,000	$ 5,800,000
Unrecognized stock-based compensation cost				3,200,000		$ 3,200,000
Unrecognized share-based compensation expense, weighted-average recognition period (in years)						1 year
Number of shares issued in offering			11,270,000
Sale of stock, price per share (in dollars per share)			$ 12.75
Gross proceeds from issuance of common stock			$ 143,700,000			$ 141,445,000	2,021,000
Net proceeds from issuance of common stock			135,100,000
Payment of underwriting fees			8,600,000
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense (gain)				$ 55,857	$ 41,907	$ 166,191	$ 41,907
Change in discount rate	15.00%	5.00%
Shares purchased by employees under ESPP				21,100	23,664	87,288	74,756
Revolving Credit Facility
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Repayments of lines of credit			$ 20,000,000