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		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Business Combinations [Abstract]
Additions to estimated acquisition earn-out payables	$ 1,787	$ 21,480	$ 2,393	$ 27,134
Payments for estimated acquisition earn-out payables	(1,485)	(9,448)	(10,562)	(14,039)
Subtotal	69,397	89,741	70,218	88,378
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	3,385	(342)	1,822	334
Interest expense accretion	672	714	1,414	1,401
Net change in earnings from estimated acquisition earn-out payables	4,057	372	3,236	1,735
Ending balance	$ 73,454	$ 90,113	$ 73,454	$ 90,113