Fair Value Measurements - Schedule of Reconciliation For Contingent Consideration Measured At Fair Value Using Significant Unobservable Inputs (Level 3) (Detail) - Fair Value, Inputs, Level 3 [Member] $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Acquisition date fair value	$ 25,491
Increase in fair value recognized in operating expenses	2,689
Contingent consideration at September 30	$ 28,180