Supplemental Gas and Oil Information - Unaudited Proved Developed and Undeveloped Oil and Gas Reserve Quantities (Details)	12 Months Ended
Dec. 31, 2014 Mcf	Dec. 31, 2013 Mcf	Dec. 31, 2012 Mcf	Dec. 31, 2011 Mcf
Reserve Quantities [Line Items]
Number of years for development plan	5 years
Percent of prior year-end proved undeveloped reserves converted to developed status (in hundredths)	7.00%	42.00%	15.00%
Natural Gas (in Mcf) [Member]
Proved Reserves [Abstract]
Beginning balance (Mcf/bbl)	811,200,000	697,200,000	758,700,000
Revisions of previous estimates (Mcf/bbl)	(220,200,000)	(112,800,000)	(158,400,000)
Extensions and discoveries (Mcf/bbl)	4,000,000	153,500,000	154,400,000
Purchase of reserves in place (Mcf/bbl)	35,900,000	133,900,000
Sale of reserves in place (Mcf/bbl)	(500,000)
Production (Mcf/bbl)	(64,300,000)	(60,600,000)	(57,500,000)
Ending balance (Mcf/bbl)	566,100,000	811,200,000	697,200,000
Proved Developed Reserves [Abstract]
Proved Developed Reserves (Mcf/bbl)	552,900,000	560,000,000	523,900,000	491,200,000
Proved Undeveloped Reserves [Abstract]
Proved Undeveloped Reserves (Mcf/bbl)	13,200,000	251,200,000	173,300,000	267,500,000
Oil and NGL (in bbl) [Member]
Proved Reserves [Abstract]
Beginning balance (Mcf/bbl)	5,617,000	6,169,000	5,344,000
Revisions of previous estimates (Mcf/bbl)	(442,000)	(1,348,000)	(122,000)
Extensions and discoveries (Mcf/bbl)	205,000	857,000	1,612,000
Purchase of reserves in place (Mcf/bbl)	157,000	556,000
Sale of reserves in place (Mcf/bbl)	(219,000)
Production (Mcf/bbl)	(587,000)	(617,000)	(665,000)
Ending balance (Mcf/bbl)	4,731,000	5,617,000	6,169,000
Proved Developed Reserves [Abstract]
Proved Developed Reserves (Mcf/bbl)	4,678,000	4,384,000	4,967,000	4,420,000
Proved Undeveloped Reserves [Abstract]
Proved Undeveloped Reserves (Mcf/bbl)	53,000	1,233,000	1,202,000	924,000
Natural Gas Equivalents (in Mcfe) [Member]
Proved Reserves [Abstract]
Beginning balance (Mcfe)	844,900,000	734,200,000	790,700,000
Revisions of previous estimates (Mcfe)	(222,700,000)	(120,800,000)	(159,000,000)
Extensions and discoveries (Mcfe)	5,200,000	158,600,000	164,000,000
Purchase of reserves in place (Mcfe)	36,800,000	137,200,000
Sale of reserves in place (Mcfe)	(1,900,000)
Production (Mcfe)	(67,800,000)	(64,300,000)	(61,500,000)
Ending balance (Mcfe)	594,500,000	844,900,000	734,200,000
Proved Developed Reserves [Abstract]
Proved Developed Reserves (Mcfe)	581,000,000	586,300,000	553,700,000	517,700,000
Proved Undeveloped Reserves [Abstract]
Proved Undeveloped Reserve (Mcfe)	13,500,000	258,600,000	180,500,000	273,000,000

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

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

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Energy measure of increase in quantity of proved reserve of oil and gas resulting from the extension of the proved acreage of previously discovered reservoir or discovery of new field with proved reserve or of new reservoir of proved reserve in old field, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

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Energy measure of proved reserve of oil and gas, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

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Energy measure of decrease in quantity of proved reserve of oil and gas resulting from production, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

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Energy measure of increase in quantity of proved reserve of oil and gas resulting from purchase of mineral in place, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

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Energy measure of increase (decrease) in quantity of proved reserve of oil and gas related to revision of a previous estimate resulting from new information, except for an increase in proved acreage, or change in economic factors, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

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Additions to proved 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 reserves or of new reservoirs of proved reserves in old fields.

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The net quantity of proved 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.

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Production of proved reserves.

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Purchase of minerals in place.

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Revisions represent changes in previous estimates of proved 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.

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Energy measure of decrease in quantity of proved reserve of oil and gas resulting from sale of mineral in place, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

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Sales of minerals in place.

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Energy measure of interests in proved developed reserves of crude oil, including condensate and natural gas liquids, natural gas, synthetic oil and gas, or other nonrenewable natural resource that is intended to be upgraded into synthetic oil and gas.

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Net quantities of an enterprise's interests in proved developed reserves of either crude oil (including condensate and natural gas liquids), natural gas, synthetic oil and gas, or other nonrenewable natural resource that is intended to be upgraded into synthetic oil and 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 are included in "net" quantities if the necessary information is available to the enterprise. "Net" quantities does not include reserves relating to interests of others in properties owned by the enterprise. The unit of measure for reserve quantities is defined as "barrels" for oil and synthetic oil reserves or "cubic feet" or "cubic meters" for natural gas and synthetic gas reserves.

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Energy measure of proved undeveloped reserves of crude oil, including condensate and natural gas liquids, natural gas, synthetic oil and gas, or other nonrenewable natural resource that is intended to be upgraded into synthetic oil and gas.

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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), natural gas, synthetic oil and gas, or other renewable natural resource that is intended to be upgraded into synthetic oil and 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 are included in "net" quantities if the necessary information is available to the enterprise. "Net" quantities does not include reserves relating to interests of others in properties owned by the enterprise. The unit of measure for reserve quantities is defined as "barrels" for oil and synthetic oil reserves or "cubic feet" or "cubic meters" for natural gas and synthetic gas reserves.

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