Fair Value Measurements - Summary of Reconciliation of Recurring Fair Value Measurements (Detail) - Deferred Consideration Obligation [Member] - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 172,746	$ 172,746
Net realized losses/(gains)	2,715	2,715
Net unrealized losses/(gains)	(9,898)	(9,898)
Settlements	(2,715)	(2,715)
Ending balance	$ 162,848	$ 162,848