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	Dec. 31, 2013	Dec. 31, 2012
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Capital costs for undeveloped acreage	$ 196,227	$ 64,840
CO2 Delivery Pipelines and Facilities
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Exploration and development costs excluded from amortization	115,828	84,183
Wells and Facilities in Progress
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Exploration and development costs excluded from amortization	$ 9,422	$ 13,898