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Fair Value Measurements - Change in Level 3 Fair Values of Contingent Purchase Price Liability (Detail) (Level 3 [Member], USD $) In Thousands, unless otherwise specified			9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Contingent Purchase Price Payable [Member]	Sep. 30, 2012 Contingent Purchase Price Payable [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ (30,861)	$ (30,012)	$ (30,012)	$ (25,325)
Additions from business acquisitions			(4,566)	(6,812)
Payment of contingent purchase price liabilities			4,893	4,924
Change in fair value of contingencies			(1,090)	402
Change in net present value of contingencies			(86)	(143)
Ending balance	$ (30,861)	$ (30,012)	$ (30,861)	$ (26,954)