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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Business Combinations [Abstract]
Additions to estimated acquisition earn-out payables	$ 1,437	$ 2,363	$ 3,828	$ 29,496
Payments for estimated acquisition earn-out payables	(16,988)	(12,899)	(27,555)	(26,937)
Subtotal	57,896	79,577	54,660	77,842
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	2,883	(365)	4,704	(30)
Interest expense accretion	727	824	2,142	2,224
Net change in earnings from estimated acquisition earn-out payables	3,610	459	6,846	2,194
Ending balance	$ 61,506	$ 80,036	$ 61,506	$ 80,036