Supplemental CO2 and Helium Disclosures (Unaudited) (Proved CO2 and Helium Reserves) (Details)	Dec. 31, 2012 Billion_cubic_feet	Dec. 31, 2011 Billion_cubic_feet	Dec. 31, 2010 Billion_cubic_feet
Reserve Quantities [Line Items]
CO2 VPP (in Bcf)	57.1	84.7	100.2
Gulf Coast Region [Member]
Reserve Quantities [Line Items]
CO2 Reserves (in MMcf)	6,073,175	6,685,412	7,085,131
Net Revenue Interest In Co2 Reserves (in Tcf)	4.8	5.3	5.6
Rocky Mountain Region [Member]
Reserve Quantities [Line Items]
CO2 Reserves (in MMcf)	3,495,534	2,195,534	2,189,756
Helium Reserves (in MMcf)	12,712	12,004	7,159
Net Revenue Interest In Co2 Reserves (in Tcf)	2.9	1.6	0.9