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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Business Combinations [Abstract]
Document Period End Date			Jun. 30, 2018
Beginning balance	$ 40,600	$ 57,408	$ 36,175	$ 63,821
Additions to estimated acquisition earn-out payables	17,549	493	22,071	282
Payments for estimated acquisition earn-out payables	(6,028)	(5,547)	(8,591)	(15,777)
Subtotal	52,121	52,354	49,655	48,326
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	(189)	4,851	1,873	8,186
Interest expense accretion	608	738	1,012	1,431
Net change in earnings from estimated acquisition earn-out payables	419	5,589	2,885	9,617
Ending balance	$ 52,540	$ 57,943	$ 52,540	$ 57,943