Total Net Product Revenue Concentrations Attributed to Largest Customers (Detail) - Customer Concentration Risk - Net Product Revenue	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Concentration Risk [Line Items]
Concentration risk, percentage	51.00%	57.00%	53.00%	53.00%
Customer A
Concentration Risk [Line Items]
Concentration risk, percentage	18.00%	17.00%	18.00%	17.00%
Customer B
Concentration Risk [Line Items]
Concentration risk, percentage	7.00%	6.00%	12.00%	6.00%
Customer C
Concentration Risk [Line Items]
Concentration risk, percentage	12.00%	14.00%	12.00%	13.00%
Customer D
Concentration Risk [Line Items]
Concentration risk, percentage	10.00%	10.00%	9.00%	10.00%
Customer E
Concentration Risk [Line Items]
Concentration risk, percentage	4.00%	10.00%	2.00%	7.00%