Accrued Restructuring Obligations and Related Activity (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Restructuring Cost and Reserve [Line Items]
Accrued obligations, beginning balance		$ 757	$ 763	$ 763
Restructuring expense	$ 625	1,306	2,062	2,886
Settlement of obligations		(1,861)		(2,892)
Accrued obligations, ending balance		202		757
Severance and Benefits
Restructuring Cost and Reserve [Line Items]
Accrued obligations, beginning balance		539	395	395
Restructuring expense	277	948	277	931
Settlement of obligations		(1,353)		(787)
Accrued obligations, ending balance		134		539
Lease Abandonment Costs
Restructuring Cost and Reserve [Line Items]
Accrued obligations, beginning balance		182	368	368
Restructuring expense	105	125	810	900
Settlement of obligations		(247)		(1,086)
Accrued obligations, ending balance		60		182
Other Costs
Restructuring Cost and Reserve [Line Items]
Accrued obligations, beginning balance		36
Restructuring expense	$ 243	233	$ 975	1,055
Settlement of obligations		(261)		(1,019)
Accrued obligations, ending balance		$ 8		$ 36