Restructuring Charges - Roll Forward of Accrued Restructuring Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring Reserve [Roll Forward]
Accrued at January 1, 2017	$ 1,210
Restructuring charges	0	$ 1,731	$ 1,642
Payments	(959)
Non-cash settlement	(228)
Accrued at December 31, 2017	23	1,210
Severance expenses
Restructuring Reserve [Roll Forward]
Accrued at January 1, 2017	901
Payments	(901)
Non-cash settlement	0
Accrued at December 31, 2017	0	901
Write-downs and retirements of fixed assets
Restructuring Reserve [Roll Forward]
Accrued at January 1, 2017	228
Payments	0
Non-cash settlement	(228)
Accrued at December 31, 2017	0	228
Lease cancellations
Restructuring Reserve [Roll Forward]
Accrued at January 1, 2017	81
Payments	(58)
Non-cash settlement	0
Accrued at December 31, 2017	$ 23	$ 81