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, 2016	Mar. 31, 2015
Business Combinations [Abstract]
Additions to estimated acquisition earn-out payables	$ 606	$ 5,653
Payments for estimated acquisition earn-out payables	(9,077)	(4,590)
Subtotal	69,916	76,346
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	(1,563)	677
Interest expense accretion	742	686
Net change in earnings from estimated acquisition earn-out payables	(821)	1,363
Ending balance	$ 69,095	$ 77,709