Commitments and Contingencies--Commitments (Details) (C O 2 Purchase Contract, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013 MMcf supplier contract	Dec. 31, 2012	Dec. 31, 2011
Long-term Purchase Commitment [Line Items]
Number of CO2 purchase contracts	4
Number of suppliers	4
Commitment period	10 years
Potential purchase volume rate reduction	100.00%
Notice required for purchase volume rate reduction	6 months
Average volume purchased (MMcf/d)	73
Purchases under CO2 contracts	$ 3,849	$ 3,860	$ 5,011
2014	2,506
2015	2,552
2016	1,289
2017	1,289
2018	2,008
2019 and thereafter	10,186
Total estimated minimum purchase obligations under CO2 contracts	$ 19,830
Minimum
Long-term Purchase Commitment [Line Items]
Contract term	P2Y
Maximum
Long-term Purchase Commitment [Line Items]
Contract term	P20Y
First Contract Year | Minimum
Long-term Purchase Commitment [Line Items]
Minimum volume required (MMcf/d)	24
Remaining Contract Term
Long-term Purchase Commitment [Line Items]
Average volume purchased (MMcf/d)	72
Remaining Contract Term | Minimum
Long-term Purchase Commitment [Line Items]
Minimum volume required (MMcf/d)	35
Fixed Price [Member]
Long-term Purchase Commitment [Line Items]
Number of CO2 purchase contracts	1
Variable Price [Member]
Long-term Purchase Commitment [Line Items]
Number of CO2 purchase contracts	2