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Business Combinations (Estimated Acquisition Earn-Out Payables) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Business Combinations [Abstract]
Balance as of January 1	$ 29,608	$ 7,354
Additions to estimated acquisition earn-out payables	30,525	25,090	7,226
Payments for estimated acquisition earn-out payables	(10,212)	(1,162)
Change in fair value on estimated acquisition earn-out payables	(4,043)	(2,606)
Interest expense accretion	1,837	932	128
Net change in earnings from estimated acquisition earn-out payables	(2,206)	(1,674)
Balance as of December 31	$ 47,715	$ 29,608	$ 7,354

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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