Oil and Gas Operations (Unaudited) (Oil and Gas Operations, Activities) (Details)	12 Months Ended
	Dec. 31, 2013 MMBoe	Dec. 31, 2012 MMBoe	Dec. 31, 2011 MMBoe
Reserve Quantities [Line Items]
Revisions of previous estimates	4.6	(42.1)	(5.8)
Purchases	0.2	12.4	20.8
Extensions and discoveries	36.8	57.1	45.6
Sales	14.8
Percentage of undeveloped portion of proved reserves	45.00%	59.00%	69.00%
Percentage of developed portion of proved reserves	55.00%	41.00%	31.00%
Extension Drilling
Reserve Quantities [Line Items]
Extensions and discoveries	21.6	45.6	41.1
Exploratory Drilling
Reserve Quantities [Line Items]
Extensions and discoveries	15.2	11.5	4.5
Price Related Revisions
Reserve Quantities [Line Items]
Revisions of previous estimates	7
No Longer Expected to be Drilled
Reserve Quantities [Line Items]
Revisions of previous estimates	(5.3)
No Longer Expected to Be Drilled Beyond Five Years
Reserve Quantities [Line Items]
Revisions of previous estimates	(4.6)
Black Warrior Basin
Reserve Quantities [Line Items]
Revisions of previous estimates		(5.1)	(0.3)
Sales	14.8
Black Warrior Basin | Price Related Revisions
Reserve Quantities [Line Items]
Revisions of previous estimates			(0.7)
Black Warrior Basin | Other Revisions
Reserve Quantities [Line Items]
Revisions of previous estimates			(0.4)
Black Warrior Basin | Change in Year-End Pricing Revisions
Reserve Quantities [Line Items]
Revisions of previous estimates		(5.9)
San Juan Basin
Reserve Quantities [Line Items]
Revisions of previous estimates	2.2	(19.7)	(2.6)
Extensions and discoveries	2.3	0.9	5.9
Number of well locations	30	6	53
San Juan Basin | Price Related Revisions
Reserve Quantities [Line Items]
Revisions of previous estimates	5.9		1.3
San Juan Basin | No Longer Expected to be Drilled
Reserve Quantities [Line Items]
Revisions of previous estimates	(4.6)
San Juan Basin | Change in Year-End Pricing Revisions
Reserve Quantities [Line Items]
Revisions of previous estimates		(22.5)
San Juan Basin | Well Performance
Reserve Quantities [Line Items]
Revisions of previous estimates			(3.9)
Permian Basin
Reserve Quantities [Line Items]
Revisions of previous estimates	(1.2)	(15.8)	(3.1)
Extensions and discoveries	34	56.1	39.6
Number of well locations	262	422	395
Permian Basin | Price Related Revisions
Reserve Quantities [Line Items]
Revisions of previous estimates	0.4	(1.0)	1.4
Permian Basin | No Longer Expected to be Drilled
Reserve Quantities [Line Items]
Revisions of previous estimates	(0.7)