Asset Retirement Obligations (Schedule of Changes in Asset Retirement Obligations) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Beginning of period total			$ 235,110
Accretion expense	6,266	3,472	18,464	10,031
Liabilities assumed in acquisitions			10,355
Liabilities incurred			1,187
Revisions			63,462
Liabilities associated with assets sold	(3,965)		(3,965)
Liabilities settled			(36,843)
End of period total	287,770		287,770
Less: End of period, current portion	47,482		47,482		30,179
End of period, noncurrent portion	240,288		240,288		204,931
Parent Company [Member]
Accretion expense	5,077	2,185	15,027	6,359
Less: End of period, current portion	42,491		42,491		23,982
End of period, noncurrent portion	$ 198,936		$ 198,936		$ 187,790