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, 2019	Mar. 31, 2018
Business Combinations [Abstract]
Document Period End Date	Mar. 31, 2019
Beginning balance	$ 89,924	$ 36,175
Additions to estimated acquisition earn-out payables	18,155	4,524
Payments for estimated acquisition earn-out payables	(579)	(2,565)
Subtotal	107,500	38,134
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	50	2,062
Interest expense accretion	1,160	404
Net change in earnings from estimated acquisition earn-out payables	1,210	2,466
Ending balance	$ 108,710	$ 40,600