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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Business Combinations [Abstract]
Additions to estimated acquisition earn-out payables	$ 1,050	$ 1,437	$ 1,332	$ 3,828
Payments for estimated acquisition earn-out payables	(23,511)	(16,988)	(39,288)	(27,555)
Subtotal	35,482	57,896	25,865	54,660
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	(1,784)	2,883	6,402	4,704
Interest expense accretion	476	727	1,907	2,142
Net change in earnings from estimated acquisition earn-out payables	(1,308)	3,610	8,309	6,846
Ending balance	$ 34,174	$ 61,506	$ 34,174	$ 61,506