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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Business Combinations [Abstract]
Additions to estimated acquisition earn-out payables	$ 493	$ 1,787	$ 282	$ 2,393
Payments for estimated acquisition earn-out payables	(5,547)	(1,485)	(15,777)	(10,562)
Subtotal	52,354	69,397	48,326	70,218
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	4,851	3,385	8,186	1,822
Interest expense accretion	738	672	1,431	1,414
Net change in earnings from estimated acquisition earn-out payables	5,589	4,057	9,617	3,236
Ending balance	$ 57,943	$ 73,454	$ 57,943	$ 73,454