Property and Equipment (Summary of Net Property and Equipment) (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
Property, Plant and Equipment [Abstract]
Proved properties	$ 9,782,337,000		$ 8,945,326,000
Unevaluated properties	918,406,000		780,481,000
Total	10,700,743,000		9,725,807,000
Accumulated depletion and depreciation of oil and natural gas properties	(3,679,883,000)		(3,219,500,000)
Net oil and natural gas properties	7,020,860,000		6,506,307,000
CO2 properties	1,162,538,000		1,117,167,000
Other property and equipment	468,051,000		466,969,000
Property, Plant and Equipment [Line Items]
Total	2,269,564,000		2,209,560,000
Accumulated depletion and depreciation	(4,248,652,000)		(3,668,225,000)
Net property and equipment	10,352,244,000		9,851,278,000
CO2 pipelines not placed in service	98,500,000
CO2 properties [Member]
Property, Plant and Equipment [Line Items]
Accumulated depletion and depreciation	(183,646,000)		(150,968,000)
Net property and equipment	978,892,000		966,199,000
Pipelines and plants [Member]
Property, Plant and Equipment [Line Items]
CO2 pipelines	1,733,562,000	[1]	1,681,774,000
Plants	536,002,000		527,786,000
Total	2,269,564,000		2,209,560,000
Accumulated depletion and depreciation	(182,385,000)		(134,697,000)
Net property and equipment	2,087,179,000		2,074,863,000
Other property and equipment [Member]
Property, Plant and Equipment [Line Items]
Accumulated depletion and depreciation	(202,738,000)		(163,060,000)
Net property and equipment	$ 265,313,000		$ 303,909,000

[1] Amount includes $98.5 million of CO2 pipelines at December 31, 2014 that were under construction and not subject to depreciation during 2014.