Summary of Restructuring Costs by Function (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Restructuring Cost And Reserve [Line Items]
Total restructuring expenses, Cash	$ 3,020	$ 3,651
Total restructuring expenses, Non-cash		911
Total restructuring expenses	3,020	4,562
Research and Development [Member]
Restructuring Cost And Reserve [Line Items]
Total restructuring expenses, Cash	188	1,631
Total restructuring expenses, Non-cash		382
Total restructuring expenses	188	2,013
Selling, General and Administrative [Member]
Restructuring Cost And Reserve [Line Items]
Total restructuring expenses, Cash	2,832	2,020
Total restructuring expenses, Non-cash		529
Total restructuring expenses	$ 2,832	$ 2,549