Fair value measurements - Roll-Forward of Fair Value of the Company's Contingent Consideration Obligations (Detail) - Significant unobservable inputs (Level 3) [Member] - Contingent consideration obligations [Member] $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 2,231
Changes in fair value	843
Ending balance	$ 3,074