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
	Mar. 31, 2018	Mar. 31, 2017
Business Combinations [Abstract]
Beginning balance	$ 36,175	$ 63,821
Additions to estimated acquisition earn-out payables	4,524	(211)
Payments for estimated acquisition earn-out payables	(2,565)	(10,230)
Subtotal	38,134	53,380
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	2,062	3,335
Interest expense accretion	404	693
Net change in earnings from estimated acquisition earn-out payables	2,466	4,028
Ending balance	$ 40,600	$ 57,408