Asset Retirement Obligations (Carrying Value of Asset Retirement Obligations) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Asset Retirement Obligations
Beginning balance as of January 1	$ 344,180	[1]	$ 341,838	[1]
Additions	59,847		47,426	[1]
Revisions in estimated cash flows, net of settlements	6,641		(70,755)	[1]
Accretion expense	25,056		25,671	[1]
Balance as of December 31	$ 435,724		$ 344,180	[1]

[1] December 31, 2011 balances have been revised to reflect purchase accounting measurement period adjustments.