Business Combinations - Additions, Payments, and Net Changes, as well as Interest Expense Accretion on Estimated Acquisition Earn-Out Payables (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Business Combinations [Abstract]
Beginning balance	$ 75,283	$ 43,058
Additions to estimated acquisition earn-out payables	5,653	280
Payments for estimated acquisition earn-out payables	(4,590)	(615)
Subtotal	76,346	42,723
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	(677)	(5,603)
Interest expense accretion	686	480
Net change in earnings from estimated acquisition earn-out payables	1,363	6,083
Ending balance	$ 77,709	$ 48,806