Restructuring Expenses (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]
Restructuring expenses	$ 111	$ 899	$ 2,886
Lease Abandonment Costs
Restructuring Cost and Reserve [Line Items]
Restructuring expenses	68	516	900
Other Costs
Restructuring Cost and Reserve [Line Items]
Restructuring expenses	$ 43	$ 383	$ 1,055