Share-based Compensation - Additional Information (Detail) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares issued under stock purchase plan, stock option exercises and restricted stock	50,928	93,486	257,883	384,761