Nature of operations and summary of significant accounting policies Nature of Operations and Summary of Significant Accounting Policies--Oil and Natural Gas Properties (Details) (USD $) In Thousands, unless otherwise specified			1 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Apr. 30, 2014 Subsequent Event Delaware Basin, Ft. Worth Basin, and Ark-La-Tex Scout Energy Group I, LP, RKI Exploration and Production, LLC, and RAM Energy, LLC contract	Mar. 31, 2014 Uncompleted Wells Equipment and Facilities	Dec. 31, 2013 Uncompleted Wells Equipment and Facilities	Mar. 31, 2014 CO2 Delivery Pipelines and Facilities	Dec. 31, 2013 CO2 Delivery Pipelines and Facilities
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Capital costs for undeveloped acreage	$ 218,369	$ 196,227
Exploration and development costs excluded from amortization				10,985	9,422	117,018	115,828
Number of purchase and sale agreements			3
Price of divested properties			201,150
Proceeds from performance deposits			$ 17,015