Commitments and Contingencies (Details) (USD $)	1 Months Ended	12 Months Ended
	Jul. 31, 2009	Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2010
Operating leases
Rent expense related to lease commitments		$ 3,000,000		$ 2,600,000	$ 2,600,000
Number of drilling rigs committed		7
Operating Leases, Future Minimum Payments Due
Operating Leases, Future Minimum Payments Due		18,000,000	[1]
Operating Leases, Future Minimum Payments Due, Current		5,100,000
Operating Leases, Future Minimum Payments, Due in Two Years		5,300,000
Operating Leases, Future Minimum Payments, Due in Three Years		3,600,000
Operating Leases, Future Minimum Payments, Due in Four Years		2,900,000
Operating Leases, Future Minimum Payments, Due in Five Years		1,100,000
Operating Leases, Future Minimum Payments, Due after Fifth Year		0
Recorded Unconditional Purchase Obligation and Operating Leases, Future Minimum Payments Due
Unrecorded Unconditional Purchase Obligation and Operating Leases, Future Minimum Payments Due		296,000,000
Unrecorded Unconditional Purchase Obligation Due and Operating Leases, Future Minimum Payments Due, Current		61,700,000
Unrecorded Unconditional Purchase Obligation Due and Operating Leases, Future Minimum Payments Due with Second Year		41,900,000
Unrecorded Unconditional Purchase Obligation Due and Operating Leases, Future Minimum Payments Due with Third Year		39,300,000
Unrecorded Unconditional Purchase Obligation Due and Operating Leases, Future Minimum Payments Due with Fourth Year		38,500,000
Unrecorded Unconditional Purchase Obligation Due and Operating Leases, Future Minimum Payments Due within Fifth Year		34,500,000
Unrecorded Unconditional Purchase Obligation Due and Operating Leases, Future Minimum Payments Due after Fifth Year		80,100,000
E. Texas Gathering System
Gas gathering system, costs incurred		6,873,000		13,038,000	19,896,000
Corporate and Field Offices
Commitments and Contingencies
Operating Lease, in Years		5 years
Vehicles
Commitments and Contingencies
Operating Lease, in Years		4 years
Airplane
Commitments and Contingencies
Operating Lease, in Years		10 years
Drilling rig commitments
Unrecorded Unconditional Purchase Obligation Payment Schedule
Unrecorded Unconditional Purchase Obligation		9,400,000	[2]
Unrecorded Unconditional Purchase Obligation Due, Current		9,400,000	[2]
Unrecorded Unconditional Purchase Obligation Due in Second Year		0	[2]
Unrecorded Unconditional Purchase Obligation Due in Third Year		0	[2]
Unrecorded Unconditional Purchase Obligation Due in Fourth Year		0	[2]
Unrecorded Unconditional Purchase Obligation Due in Fifth Year		0	[2]
Unrecorded Unconditional Purchase Obligation Due after Fifth Year		0	[2]
Other commitments
Unrecorded Unconditional Purchase Obligation Payment Schedule
Unrecorded Unconditional Purchase Obligation		23,500,000
Unrecorded Unconditional Purchase Obligation Due, Current		16,200,000	[3]
Unrecorded Unconditional Purchase Obligation Due in Second Year		2,900,000	[3]
Unrecorded Unconditional Purchase Obligation Due in Third Year		2,300,000	[3]
Unrecorded Unconditional Purchase Obligation Due in Fourth Year		2,100,000	[3]
Unrecorded Unconditional Purchase Obligation Due in Fifth Year		0	[3]
Unrecorded Unconditional Purchase Obligation Due after Fifth Year		0	[3]
Firm natural gas transportation contracts
Unrecorded Unconditional Purchase Obligation Payment Schedule
Unrecorded Unconditional Purchase Obligation		245,100,000	[4]
Unrecorded Unconditional Purchase Obligation Due, Current		31,000,000
Unrecorded Unconditional Purchase Obligation Due in Second Year		33,700,000
Unrecorded Unconditional Purchase Obligation Due in Third Year		33,400,000
Unrecorded Unconditional Purchase Obligation Due in Fourth Year		33,500,000
Unrecorded Unconditional Purchase Obligation Due in Fifth Year		33,400,000
Unrecorded Unconditional Purchase Obligation Due after Fifth Year		80,100,000
East Texas
E. Texas Gathering System
Proceeds from gas gathering system, financing obligation	18,400,000
Net book value of property	16,700,000
Gathering Agreements, Total Cost Incurred		$ 3,200,000		$ 5,300,000	$ 6,700,000
Gross | Uinta
Recorded Unconditional Purchase Obligation and Operating Leases, Future Minimum Payments Due
Minimum Contractual Daily Crude Oil Sales		5,000
Uinta Crude Oil Sales Contract
Gross oil production average (in Bbl/D)		4,420

[1] Includes operating leases related to office facilities, vehicles, rail cars and aircraft.

[2] Excludes obligations related to rigs drilling on a well-by-well basis that can be released after drilling the current well without penalty.

[3] Includes primarily obligations related to natural gas purchases, cogeneration facility management services, California carbon allowance forward purchase contracts and equipment rentals.

[4] The Company enters into certain firm commitments to transport natural gas production to market and to transport natural gas for use in its cogeneration and conventional steam generation facilities. These commitments generally require a minimum monthly charge regardless of whether the contracted capacity is used or not.