Share-based Compensation - Share-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total share-based compensation	$ 537	$ 887	$ 1,794	$ 1,845
Stock Option Awards [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total share-based compensation	331	723	1,111	1,347
Restricted Stock Units [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total share-based compensation	$ 206	$ 164	$ 683	$ 498