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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Business Combinations [Abstract]
Document Period End Date			Sep. 30, 2018
Beginning balance		$ 57,943	$ 36,175	$ 63,821
Additions to estimated acquisition earn-out payables	$ 29,646	1,050	51,717	1,332
Payments for estimated acquisition earn-out payables	(16,521)	(23,511)	(25,112)	(39,288)
Subtotal	65,665	35,482	62,780	25,865
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	(928)	(1,784)	945	6,402
Interest expense accretion	571	476	1,583	1,907
Net change in earnings from estimated acquisition earn-out payables	(357)	(1,308)	2,528	8,309
Ending balance	$ 65,308	$ 34,174	$ 65,308	$ 34,174