Commitments and Contingencies--Narrative (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended			1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 30, 2014 Lease
Loss Contingencies [Line Items]
Letters of credit outstanding	$ 920	$ 920
Interest Paid	112,196	107,122	103,350
Leases, Operating [Abstract]
Rent expense	11,088	8,967	8,144
Maximum remaining term of operating leases	5 years
Minimum [Member] | Pending Litigation [Member]
Litigation and Claims [Abstract]
Damages sought	5,000
Letter of Credit [Member]
Loss Contingencies [Line Items]
Interest Paid	0	0	0
CO2 Compressor [Member]
Leases, Operating [Abstract]
Number Of Operating Leases				2
C O 2 Purchase Contract [Member]
Loss Contingencies [Line Items]
Number of short-term interim agreements	4
Number of suppliers	4
Contract term	10 years
Minimum volume required (MMcf/d)	35
Potential purchase volume rate reduction	100.00%
Notice required for purchase volume rate reduction	6 months
Average volume purchased (MMcf/d)	68
Purchases under CO2 contracts	4,890	3,849	3,860
C O 2 Purchase Contract [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Contract term	2 years
Average volume purchased (MMcf/d)	68
C O 2 Purchase Contract [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Contract term	20 years
Average volume purchased (MMcf/d)	71,000
Fixed Price [Member] | C O 2 Purchase Contract [Member]
Loss Contingencies [Line Items]
Number of short-term interim agreements	1
Variable Price [Member] | C O 2 Purchase Contract [Member]
Loss Contingencies [Line Items]
Number of short-term interim agreements	2
Severance Payable If Terminated Under Change In Control [Member]
Other Contingencies [Abstract]
Supplemental Unemployment Benefits, Severance Benefits	$ 10,833