Accrued Restructuring Obligations and Related Activity (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Restructuring Cost and Reserve [Line Items]
Accrued obligations, beginning balance	$ 757	$ 763	$ 763
Restructuring expense	111	899	2,886
Settlement of obligations	(338)		(2,892)
Accrued obligations, ending balance	530		757
Severance and Benefits
Restructuring Cost and Reserve [Line Items]
Accrued obligations, beginning balance	539	395	395
Restructuring expense			931
Settlement of obligations	(185)		(787)
Accrued obligations, ending balance	354		539
Lease Abandonment Costs
Restructuring Cost and Reserve [Line Items]
Accrued obligations, beginning balance	182	368	368
Restructuring expense	68	516	900
Settlement of obligations	(109)		(1,086)
Accrued obligations, ending balance	141		182
Other Costs
Restructuring Cost and Reserve [Line Items]
Accrued obligations, beginning balance	36
Restructuring expense	43	$ 383	1,055
Settlement of obligations	(44)		(1,019)
Accrued obligations, ending balance	$ 35		$ 36