Asset Retirement Obligations (Schedule of Changes in Asset Retirement Obligations) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Beginning of period total			$ 235,110
Accretion expense	6,166	3,411	12,198	6,559
Liabilities incurred			334
Revisions			10,261
Liabilities associated with assets sold	(3,965)		(3,965)
Liabilities settled			(26,771)
End of period total	227,167		227,167
Less: End of period, current portion	28,162		28,162		30,179
End of period, noncurrent portion	199,005		199,005		204,931
Parent Company [Member]
Accretion expense	5,026	2,180	9,950	4,173
Less: End of period, current portion	22,219		22,219		23,982
End of period, noncurrent portion	$ 182,915		$ 182,915		$ 187,790