Derivative Financial Instruments (Details)	Mar. 31, 2013 MBbls
Provisional Oil Sales
Derivative Financial Instruments
Volumes (in MBbl)	995.1
Average price, subject to final pricing	110.45
Three-way Collars | Term April 2013 to December 2013
Derivative Financial Instruments
Volumes (in MBbl)	1,125
Weighted average deferred premium per Bbl	4.82
Weighted average floor price per Bbl	95.00
Weighted average ceiling price per Bbl	105.00
Weighted average call price per Bbl	125.00
Three-way Collars | Term January 2014 to December 2014
Derivative Financial Instruments
Volumes (in MBbl)	1,500
Weighted average deferred premium per Bbl	1.22
Weighted average floor price per Bbl	85.00
Weighted average ceiling price per Bbl	115.00
Weighted average call price per Bbl	140.00
Three-way collars | Term April 2013 to December 2013
Derivative Financial Instruments
Volumes (in MBbl)	756
Weighted average floor price per Bbl	87.50
Weighted average ceiling price per Bbl	115.00
Weighted average call price per Bbl	135.00
Three-way collars | Term January 2014 to December 2014
Derivative Financial Instruments
Volumes (in MBbl)	1,000
Weighted average floor price per Bbl	85.00
Weighted average ceiling price per Bbl	115.01
Weighted average call price per Bbl	140.00
Three-way collars | Term April 2013 to December 2013
Derivative Financial Instruments
Volumes (in MBbl)	750
Weighted average floor price per Bbl	90.00
Weighted average ceiling price per Bbl	115.39
Weighted average call price per Bbl	135.00
Three-way collars | Term April 2013 to December 2013
Derivative Financial Instruments
Volumes (in MBbl)	750
Weighted average floor price per Bbl	90.08
Weighted average ceiling price per Bbl	115.00
Weighted average call price per Bbl	135.00