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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, 2013	Mar. 31, 2012
Business Combinations [Abstract]
Beginning balance	$ 52,987	$ 47,715
Additions to estimated acquisition earn-out payables	(721)	4,714
Payments for estimated acquisition earn-out payables	(4,319)	(133)
Subtotal	47,947	52,296
Change in fair value on estimated acquisition earn-out payables	997	(970)
Interest expense accretion	525	582
Net change in earnings from estimated acquisition earn-out payables	1,522	(388)
Ending balance	$ 49,469	$ 51,908

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

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