Restructuring Charges - Additional Information (Detail) - USD ($) $ in Millions	2 Months Ended	6 Months Ended
	Dec. 31, 2017	Jun. 30, 2018
Restructuring Cost And Reserve [Line Items]
Accrued and unpaid severance costs	$ 1.5	$ 0.1
Stock-Based Compensation Expense [Member] | Chief Medical Officer [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring costs	1.0
Selling, General and Administrative Expense [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring charge	$ 3.8
Severance Costs [Member]
Restructuring Cost And Reserve [Line Items]
Severance costs paid		$ 1.4