Oil and Gas Operations (Unaudited) (Oil and Gas Operations, Activites) (Details)	12 Months Ended
	Dec. 31, 2011 mmboe	Dec. 31, 2010 mmboe	Dec. 31, 2009 mmboe
Reserve Quantities [Line Items]
Revisions of previous estimates	(5.8)	11.3	(20.7)
Purchases	20.8	25.8	16.2
Extensions and discoveries	45.6	26.8	17.7
Percentage of undeveloped portion of proved reserves	69.00%	77.00%	81.00%
Percentage of developed portion of proved reserves	31.00%	23.00%	19.00%
Extension Drilling
Reserve Quantities [Line Items]
Extensions and discoveries	41.1	26.6	17.7
Exploratory Drilling
Reserve Quantities [Line Items]
Extensions and discoveries	4.5	0.3	0.1
Black Warrior Basin
Reserve Quantities [Line Items]
Revisions of previous estimates	(0.3)	0.6	(7.6)
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		1.3	(3.4)
Black Warrior Basin | Accelerated Coal Mining Plans
Reserve Quantities [Line Items]
Revisions of previous estimates			(2.1)
San Juan Basin
Reserve Quantities [Line Items]
Revisions of previous estimates	(2.6)	11.0	(12.3)
Extensions and discoveries	5.9	6.4	6.4
Number of well locations	53	36	46
Number of sidetrack wells		1	10
San Juan Basin | Sidetrack Drilling
Reserve Quantities [Line Items]
Extensions and discoveries		1.1	1.1
San Juan Basin | Price Related Revisions
Reserve Quantities [Line Items]
Revisions of previous estimates	1.3		(11.7)
San Juan Basin | Change in Year-End Pricing Revisions
Reserve Quantities [Line Items]
Revisions of previous estimates		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	(3.1)	(1.3)	1.1
Extensions and discoveries	39.6	22.1	9.5
Number of well locations	395	271	130
Permian Basin | Price Related Revisions
Reserve Quantities [Line Items]
Revisions of previous estimates	1.4	3.0	4.2