Fair Value Measurements - Fair Value Liabilities Measured on Recurring Basis (Detail) - Fair Value Measurements Recurring [Member] - Fair Value, Inputs, Level 3 [Member] $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Additions	$ 43,500
Fair value adjustments	3,000
Contingent consideration obligation, Ending Balance	$ 46,500