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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Business Combinations [Abstract]
Additions to estimated acquisition earn-out payables	$ 21,480	$ 14,527	$ 27,134	$ 14,807
Payments for estimated acquisition earn-out payables	(9,448)	(10,814)	(14,039)	(11,429)
Subtotal	89,741	52,519	88,378	46,436
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	342	375	334	5,228
Interest expense accretion	714	552	1,401	1,032
Net change in earnings from estimated acquisition earn-out payables	372	177	1,735	6,260
Ending balance	$ 90,113	$ 52,696	$ 90,113	$ 52,696