Business Combinations - Additions, Payments, and Net Changes, as well as Interest Expense Accretion on Estimated Acquisition Earn-Out Payables (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Business Combinations [Abstract]
Beginning balance	$ 75,283	$ 43,058	$ 52,987
Additions to estimated acquisition earn-out payables	36,899	34,356	5,816
Payments for estimated acquisition earn-out payables	36,798	12,069	18,278
Subtotal	75,384	65,345	40,525
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	(2,990)	(7,375)	(570)
Interest expense accretion	13	2,563	1,963
Net change in earnings from estimated acquisition earn-out payables	3,003	9,938	2,533
Ending balance	$ 78,387	$ 75,283	$ 43,058