Accounts Receivable - Additional Information (Detail) - Distributor	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Receivables [Abstract]
Number of wholesale distributors	3	3	3
Percentage of accounts receivable from major wholesale distributors	94.00%	94.00%	93.00%