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	Sep. 30, 2014	Dec. 31, 2013
Capitalized costs of unproved properties excluded from amortization
Capital costs for undeveloped acreage	$ 240,019	$ 196,227
CO2 Delivery Pipelines and Facilities
Capitalized costs of unproved properties excluded from amortization
Exploration and development costs excluded from amortization	72,046	115,828
Uncompleted Wells Equipment and Facilities
Capitalized costs of unproved properties excluded from amortization
Exploration and development costs excluded from amortization	$ 15,430	$ 9,422