Supplemental Gas and Oil Information Proved Developed and Undeveloped Oil and Gas Reserve Quantities (Details)	12 Months Ended
Dec. 31, 2011 Bcf	Dec. 31, 2010 Bcf	Dec. 31, 2009 Bcf	Dec. 31, 2008 Bcf
Reserve Quantities [Line Items]
Number of years for development plan	5 years
Percent of prior year-end proved undeveloped reserves converted to developed status	19.00%
Natural Gas (in Bcf) [Member]
Proved Reserves Abstract
Beginning balance	738.4	649.4	646.9
Revisions - previous estimates	(153.9)	(202.3)	(27.3)
Extensions and discoveries	224.7	341.5	78.0
Production	(50.5)	(50.2)	(48.2)
Ending balance	758.7	738.4	649.4	646.9
Proved Developed Reserves Abstract
Proved Developed Reserves (Volume)	491.2	463.8	477.1	471.4
Proved Undeveloped Reserves Abstract
Proved Undeveloped Reserves (Volume)	267.5	274.6	172.3	175.5
Oil and NGL (in MMbbl) [Member]
Proved Reserves Abstract
Beginning balance	4.9	4.5	4.5
Revisions - previous estimates	(0.3)	(1.3)	(0.2)
Extensions and discoveries	1.2	2.1	0.6
Production	(0.5)	(0.4)	(0.4)
Ending balance	5.3	4.9	4.5	4.5
Proved Developed Reserves Abstract
Proved Developed Reserves (Volume)	4.4	3.2	3.1	3.1
Proved Undeveloped Reserves Abstract
Proved Undeveloped Reserves (Volume)	0.9	1.7	1.4	1.4
Natural Gas Equivalents (in Bcfe) [Member]
Proved Reserves Abstract
Beginning balance	767.5	676.3	673.9
Revisions - previous estimates	(155.5)	(210.1)	(28.3)
Extensions and discoveries	232.0	354.2	81.4
Production	(53.3)	(52.9)	(50.7)
Ending balance	790.7	767.5	676.3	673.9
Proved Developed Reserves Abstract
Proved Developed Reserves (Volume)	517.7	482.6	495.5	489.9
Proved Undeveloped Reserves Abstract
Proved Undeveloped Reserves (Volume)	273.0	284.9	180.8	184.0

The number of years established by the compnay for the development plan outlined.

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- Definition

Percent of prior year-end proved undeveloped reserves converted to developed status

+ References

No definition available.

+ Details

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- Definition

Additions to proved oil reserves that result from (1) extension of the proved acreage of previously discovered (old) reservoirs through additional drilling in periods after discovery and (2) discovery of new fields with proved oil reserves or of new reservoirs of proved oil reserves in old fields.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 19

-Paragraph 59F

-Subparagraph d

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- Definition

The net quantity of proved oil reserves as of the balance sheet date. Proved oil reserves are the estimated quantities of crude oil and natural gas liquids which geological and engineering data demonstrate with reasonable certainty to be recoverable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 19

-Paragraph 59F

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 10

-Paragraph a

-Subparagraph 2

-Article 4

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- Definition

Production of proved oil reserves

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 19

-Paragraph 59F

-Subparagraph e

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- Definition

Revisions represent changes in previous estimates of proved oil reserves, either upward or downward, resulting from new information (except for an increase in proved acreage) normally obtained from development drilling and production history or resulting from change in economic factors.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 19

-Paragraph 59F

-Subparagraph a

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- Definition

Net quantities of an enterprise's interests in proved developed reserves of either crude oil (including condensate and natural gas liquids) or natural gas as of the beginning and the end of the year. "Net" quantities of reserves include those relating to the enterprise's operating and nonoperating interests in properties. Quantities of reserves relating to royalty interests owned shall be included in "net" quantities if the necessary information is available to the enterprise. "Net" quantities shall not include reserves relating to interests of others in properties owned by the enterprise. The unit of measure for reserve quantities should be defined in the unit attribute of the instance document as "barrels" for oil reserves or "cubic_feet" or "cubic_meters" for natural gas reserves.

+ References

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Net quantities of an enterprise's interests in proved undeveloped reserves of either crude oil (including condensate and natural gas liquids) or natural gas as of the beginning and the end of the year. "Net" quantities of reserves include those relating to the enterprise's operating and nonoperating interests in properties. Quantities of reserves relating to royalty interests owned shall be included in "net" quantities if the necessary information is available to the enterprise. "Net" quantities shall not include reserves relating to interests of others in properties owned by the enterprise. The unit of measure for reserve quantities should be defined in the unit attribute of the instance document as "barrels" for oil reserves or "cubic_feet" or "cubic_meters" for natural gas reserves.

+ References

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