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		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Business Combinations [Abstract]
Additions to estimated acquisition earn-out payables	$ 2,363	$ (221)	$ 29,496	$ 14,586
Payments for estimated acquisition earn-out payables	(12,899)	(463)	(26,937)	(11,892)
Subtotal	79,577	52,012	77,842	45,752
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	365	247	(30)	4,981
Interest expense accretion	824	561	2,224	1,593
Net change in earnings from estimated acquisition earn-out payables	459	314	2,194	6,574
Ending balance	$ 80,036	$ 52,326	$ 80,036	$ 52,326