Asset Retirement Obligations (Schedule of Changes in Asset Retirement Obligations) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Beginning of period total	$ 235,110	$ 99,347
Accretion expense	28,299	15,565	15,942
Liabilities assumed in acquisition	23,541	132,109
Liabilities incurred	1,187	1,210
Revisions	24,586	22,308
Liabilities associated with assets sold	(3,965)
Liabilities settled	(53,308)	(35,429)
End of period total	255,450	235,110	99,347
Less: End of period, current portion	51,601	30,179
End of period, noncurrent portion	203,849	204,931
Parent Company [Member]
Accretion expense	23,196	10,551	9,013
Less: End of period, current portion	51,601	23,982
End of period, noncurrent portion	$ 160,466	$ 187,790