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Restructuring Activities - Summary of Charges Related to Restructuring Activities (Detail) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Restructuring Cost And Reserve [Line Items]
Accrued Restructuring Expenses				$ 921
Expenses			$ 0	4,825	$ 0
Less: Payments	$ (800)	$ (400)	$ (200)	(5,624)	$ (600)	$ (900)
Accrued Restructuring Expenses	122	921		122		921
Severance Benefits and Related Costs Due to Workforce Reduction [Member]
Restructuring Cost And Reserve [Line Items]
Accrued Restructuring Expenses				921
Expenses				4,825
Less: Payments				(5,624)
Accrued Restructuring Expenses	$ 122	$ 921		$ 122		$ 921