Fair Value Measurements - Reconciliation of the Beginning and Ending Level 3 Net Liabilities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Payment of contingent earnout consideration	$ 5,189	$ 2,214
Adjustment to estimated fair value of contingent earnout consideration (included within Selling, general and administrative expense)	252	725
Accretion of contingent earnout consideration (included within other expense, net)	(295)	(2,182)	$ (1,710)
Significant Unobservable Inputs (Level 3) [Member] | Zero2Ten and Branchbird Contingent Earnout Consideration [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	8,089	10,540
Payment of contingent earnout consideration	8,132	(3,906)
Adjustment to estimated fair value of contingent earnout consideration (included within Selling, general and administrative expense)	(252)	(725)
Accretion of contingent earnout consideration (included within other expense, net)	$ 295	2,180
Ending balance		$ 8,089	$ 10,540