Derivatives (Details)	Dec. 31, 2012 item
Derivatives
Btu factor	1,000
Commodity Derivatives | Swaps | Oil
Derivatives
Volume (in Bbls or MMBtu)	2,513,000
Commodity Derivatives | Swaps | Oil | 1st Quarter 2013
Derivatives
Volume (in Bbls or MMBtu)	665,000
Price (in dollars per barrel)	93.70
Commodity Derivatives | Swaps | Oil | 2nd Quarter 2013
Derivatives
Volume (in Bbls or MMBtu)	648,000
Price (in dollars per barrel)	93.940
Commodity Derivatives | Swaps | Oil | 3rd Quarter 2013
Derivatives
Volume (in Bbls or MMBtu)	300,000
Price (in dollars per barrel)	104.600
Commodity Derivatives | Swaps | Oil | 4th Quarter 2013
Derivatives
Volume (in Bbls or MMBtu)	300,000
Price (in dollars per barrel)	104.600
Commodity Derivatives | Swaps | Oil | 2014
Derivatives
Volume (in Bbls or MMBtu)	600,000
Price (in dollars per barrel)	99.30
Commodity Derivatives | Swaps | Gas
Derivatives
Volume (in Bbls or MMBtu)	1,480,000	[1]
Commodity Derivatives | Swaps | Gas | 1st Quarter 2013
Derivatives
Volume (in Bbls or MMBtu)	400,000	[1]
Price (in dollars per barrel)	3.34
Commodity Derivatives | Swaps | Gas | 2nd Quarter 2013
Derivatives
Volume (in Bbls or MMBtu)	390,000	[1]
Price (in dollars per barrel)	3.34
Commodity Derivatives | Swaps | Gas | 3rd Quarter 2013
Derivatives
Volume (in Bbls or MMBtu)	360,000	[1]
Price (in dollars per barrel)	3.34
Commodity Derivatives | Swaps | Gas | 4th Quarter 2013
Derivatives
Volume (in Bbls or MMBtu)	330,000	[1]
Price (in dollars per barrel)	3.34
Commodity Derivatives | Swaps | Gas | 2014
Derivatives
Volume (in Bbls or MMBtu)	0	[1]
Price (in dollars per barrel)	0

[1] One MMBtu equals one Mcf at a Btu factor of 1,000.