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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	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Business Combination, Separately Recognized Transactions [Line Items]
Beginning balance	$ 47,715	$ 29,608	$ 7,354
Additions to estimated acquisition earn-out payables	21,479	30,525	25,090
Payments for estimated acquisition earn-out payables	(17,625)	(10,212)	(1,162)
Subtotal	51,569	49,921	31,282
Change in fair value on estimated acquisition earn-out payables	(1,051)	(4,043)	(2,606)
Interest expense accretion	2,469	1,837	932
Net change in earnings from estimated acquisition earn-out payables	1,418	(2,206)	(1,674)
Ending balance	$ 52,987	$ 47,715	$ 29,608

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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