Business Combinations - Additions, Payments, and Net Changes, as well as Interest Expense Accretion on Estimated Acquisition Earn-Out Payables (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Business Combinations [Abstract]
Beginning balance	$ 43,058,000	$ 52,987,000	$ 47,715,000
Additions to estimated acquisition earn-out payables	34,356,000	5,816,000	21,479,000
Payments for estimated acquisition earn-out payables	(12,069,000)	(18,278,000)	(17,625,000)
Subtotal	65,345,000	40,525,000	51,569,000
Change in fair value on estimated acquisition earn-out payables	7,375,000	570,000	(1,051,000)
Interest expense accretion	2,563,000	1,963,000	2,469,000
Net change in earnings from estimated acquisition earn-out payables	9,938,000	2,533,000	1,418,000
Ending balance	$ 75,283,000	$ 43,058,000	$ 52,987,000