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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Business Combinations [Abstract]
Document Period End Date			Jun. 30, 2019
Beginning balance	$ 108,710	$ 40,600	$ 89,924	$ 36,175
Additions to estimated acquisition earn-out payables	5,811	17,549	23,966	22,071
Payments for estimated acquisition earn-out payables	(6,685)	(6,028)	(7,264)	(8,591)
Subtotal	107,836	52,121	106,626	49,655
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	(4,350)	(189)	(4,301)	1,873
Interest expense accretion	1,490	608	2,651	1,012
Net change in earnings from estimated acquisition earn-out payables	(2,860)	419	(1,650)	2,885
Ending balance	$ 104,976	$ 52,540	$ 104,976	$ 52,540