Business Combinations - Additions, Payments, and Net Changes, as well as Interest Expense Accretion on Estimated Acquisition Earn-Out Payables (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Business Acquisition [Line Items]
Maximum Future Contingency payments Acquisitions	$ 106,400
Additions to estimated acquisition earn-out payables	(211)	$ 606
Payments for estimated acquisition earn-out payables	(10,230)	(9,077)
Subtotal	53,380	69,916
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	3,335	(1,563)
Interest expense accretion	693	742
Net change in earnings from estimated acquisition earn-out payables	4,028	(821)
Ending balance	$ 57,408	$ 69,095