Oil and Gas Operations (Unaudited) (Oil and Gas Operations, Activites) (Details)	12 Months Ended
	Dec. 31, 2012 MMBoe	Dec. 31, 2011 MMBoe	Dec. 31, 2010 MMBoe
Reserve Quantities [Line Items]
Revisions of previous estimates	42.1	5.8	(11.3)
Purchases	12.4	20.8	25.8
Extensions and discoveries	57.1	45.6	26.8
Percentage of undeveloped portion of proved reserves	59.00%	69.00%	77.00%
Percentage of developed portion of proved reserves	41.00%	31.00%	23.00%
Extension Drilling
Reserve Quantities [Line Items]
Extensions and discoveries	45.6	41.1	26.6
Exploratory Drilling
Reserve Quantities [Line Items]
Extensions and discoveries	11.5	4.5	0.3
Black Warrior Basin
Reserve Quantities [Line Items]
Revisions of previous estimates	5.1	0.3	(0.6)
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)	0.7
Black Warrior Basin | Change in Year-End Pricing Revisions
Reserve Quantities [Line Items]
Revisions of previous estimates	5.9		(1.3)
San Juan Basin
Reserve Quantities [Line Items]
Revisions of previous estimates	19.7	2.6	(11.0)
Extensions and discoveries	0.9	5.9	6.4
Number of well locations	6	53	36
Number of sidetrack wells			1
San Juan Basin | Sidetrack Drilling
Reserve Quantities [Line Items]
Extensions and discoveries			1.1
San Juan Basin | Price Related Revisions
Reserve Quantities [Line Items]
Revisions of previous estimates		(1.3)
San Juan Basin | Change in Year-End Pricing Revisions
Reserve Quantities [Line Items]
Revisions of previous estimates	22.5		(7.6)
San Juan Basin | Well Performance
Reserve Quantities [Line Items]
Revisions of previous estimates		3.9	(8.2)
San Juan Basin | SEC Five-Year Development Rule Revisions
Reserve Quantities [Line Items]
Revisions of previous estimates			5.3
Permian Basin
Reserve Quantities [Line Items]
Revisions of previous estimates	15.8	3.1	1.3
Extensions and discoveries	56.1	39.6	22.1
Number of well locations	422	395	271
Permian Basin | Price Related Revisions
Reserve Quantities [Line Items]
Revisions of previous estimates	1.0	(1.4)	(3.0)