Restructuring Activities - Summary of Charges Related to Restructuring Activities (Detail) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2018
Restructuring Cost And Reserve [Line Items]
Accrued Restructuring Expenses	$ 921		$ 628	$ 628
Expenses	347
Less: Payments	(1,059)	$ (400)	(377)
Accrued Restructuring Expenses	209	921	251	921
Severance Benefits and Related Costs Due to Workforce Reduction [Member]
Restructuring Cost And Reserve [Line Items]
Accrued Restructuring Expenses	921		628	628
Expenses	347
Less: Payments	(1,059)		(377)
Accrued Restructuring Expenses	$ 209	$ 921	$ 251	$ 921