Restructuring Activities- Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Restructuring Cost And Reserve [Line Items]
Restructuring activity, announcement date		Nov. 07, 2018
Restructuring expenses		$ 347
Payments for restructuring		1,059	$ 400	$ 377
Accrued remaining payments for restructuring		209	$ 921	$ 251	$ 921	$ 628
Scenario, Forecast
Restructuring Cost And Reserve [Line Items]
Payments for restructuring	$ 200
One-time Termination Benefits [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring expenses		300			1,300
One-time Termination Benefits [Member] | Research and Development Expense [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring expenses		200			1,000
One-time Termination Benefits [Member] | General and Administrative Expense [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring expenses		$ 100			$ 300