Property and Equipment (Summary of Net Property and Equipment) (Details) (USD $)	Dec. 31, 2013		Dec. 31, 2012
Property, Plant and Equipment [Abstract]
Proved properties	$ 8,945,326,000		$ 6,963,211,000
Unevaluated properties	780,481,000		809,154,000
Total	9,725,807,000		7,772,365,000
Accumulated depletion and depreciation of oil and natural gas properties	(3,219,500,000)		(2,827,256,000)
Net oil and natural gas properties	6,506,307,000		4,945,109,000
CO2 properties	1,117,167,000		1,032,653,000
CO2 pipelines	1,681,774,000	[1]	1,632,255,000
Plants	527,786,000		402,871,000
Total	2,209,560,000		2,035,126,000
Other property and equipment	466,969,000		417,207,000
Property, Plant and Equipment [Line Items]
Accumulated depletion and depreciation	(3,668,225,000)		(3,180,241,000)
Net property and equipment	9,851,278,000		8,077,110,000
CO2 pipelines not placed in service	48,400,000
CO2 properties [Member]
Property, Plant and Equipment [Line Items]
Accumulated depletion and depreciation	(150,968,000)		(119,784,000)
Net property and equipment	966,199,000		912,869,000
Pipelines and plants [Member]
Property, Plant and Equipment [Line Items]
Accumulated depletion and depreciation	(134,697,000)		(99,185,000)
Net property and equipment	2,074,863,000		1,935,941,000
Other property and equipment [Member]
Property, Plant and Equipment [Line Items]
Accumulated depletion and depreciation	(163,060,000)		(134,016,000)
Net property and equipment	$ 303,909,000		$ 283,191,000

[1] Amounts include $48.4 million of CO2 pipelines at December 31, 2013 that were under construction and not subject to depreciation during 2013.