Fair Value Measurements - Change in Level 3 Fair Values of Contingent Purchase Price Liabilities (Detail) - Level 3 [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ (33,709)
Ending balance	(37,574)	$ (33,709)
Contingent Purchase Price Liabilities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	(33,709)	(24,817)
Additions from business acquisitions	(19,291)	(21,088)
Settlement of contingent purchase price payable	13,932	11,202
Change in fair value of contingency	2,128	1,342
Change in net present value of contingency	(634)	(348)
Ending balance	$ (37,574)	$ (33,709)