Fair Value Measurements - Change in Level 3 Fair Values of Contingent Purchase Price Liabilities (Detail) - Level 3 [Member] - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ (37,574)
Ending balance	(39,896)
Contingent Purchase Price Liabilities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	(37,574)	$ (33,709)
Additions from business acquisitions	(12,361)	(17,526)
Settlement of contingent purchase price liabilities	13,329	11,644
Change in fair value of contingencies	(2,574)	651
Change in net present value of contingencies	(716)	(430)
Ending balance	$ (39,896)	$ (39,370)