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Acquisitions (Details) (USD $) | 0 Months Ended | 1 Months Ended | 3 Months Ended | 12 Months Ended | 0 Months Ended | 23 Months Ended |  | 0 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 24, 2012 | Feb. 28, 2011 | Mar. 31, 2013 | Mar. 31, 2012 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Feb. 15, 2013 Utica Shale [Member] well acre | Dec. 21, 2012 Utica Shale [Member] acre | Dec. 18, 2012 Utica Shale [Member] | Dec. 19, 2012 Utica Shale [Member] acre | Dec. 24, 2012 Utica Shale [Member] acre | Dec. 31, 2012 Utica Shale [Member] acre wells | Dec. 17, 2012 Utica Shale [Member] acre | Feb. 15, 2013 Common Stock [Member] Public Offering [Member] | Jan. 07, 2013 Common Stock [Member] Public Offering [Member] | Dec. 24, 2012 Common Stock [Member] Public Offering [Member] | Jan. 07, 2013 Common Stock [Member] Public Offering [Member] | Dec. 05, 2011 Common Stock [Member] Public Offering [Member] | Jul. 15, 2011 Common Stock [Member] Public Offering [Member] | Mar. 30, 2011 Common Stock [Member] Public Offering [Member] | May 25, 2010 Common Stock [Member] Public Offering [Member] | May 25, 2010 Common Stock [Member] Public Offering [Member]
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period allowed for refusal |  | 6 months |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Affiliates participation ratio |  |  | 50.00% |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gas and oil areage, undeveloped, net |  |  |  |  |  |  |  | 22,000 | 37,000 |  | 7,000 | 37,000 | 106,000 | 30,000 |  |  |  |  |  |  |  |  |
Cash price of ownership interest |  |  |  |  |  |  |  | $ 220,000,000 | $ 70,000,000 | $ 302,000,000 | $ 372,000,000 |  | $ 619,600,000 |  |  |  |  |  |  |  |  |  |
Gas and oil areage, undeveloped, gross (acres) |  |  |  |  |  |  |  |  |  |  |  |  | 137,000 |  |  |  |  |  |  |  |  |  |
Amount put in escrow |  |  |  |  |  |  |  | 33,600,000 |  |  | 53,900,000 |  |  |  |  |  |  |  |  |  |  |  |
Stock option, portion attributable to underwriters (shares) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 900,000 | 750,000 |  |  |  |  | 187,022 |
Number Of Wells Owned |  |  |  |  |  |  |  | 14 |  |  |  |  | 14 |  |  |  |  |  |  |  |  |  |
Proceeds from issuance of common stock, net of offering costs, and exercise of stock options | $ 427,900,000 |  | $ 357,640,000 |  | $ 427,091,000 | $ 307,154,000 | $ 21,879,000 | $ 325,800,000 |  |  |  | $ 460,700,000 |  |  | $ 325,800,000 |  | $ 427,900,000 | $ 460,700,000 | $ 128,000,000 | $ 94,700,000 | $ 84,300,000 |  | $ 21,600,000
Increase (Decrease) In Lease Interest, Percentage |  |  |  |  |  |  |  | 16.20% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Lease Interest, Percentage |  |  |  |  |  |  |  | 93.80% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of proposed future wells excluded from acquisition |  |  |  |  |  |  |  | 16 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition The designation of funds furnished by a borrower to a lender to assure future payments of the borrower's real estate taxes and insurance obligations with respect to a mortgaged property. Escrow deposits may be made for a variety of other purposes such as earnest money and contingent payments. This element excludes replacement reserves which are an escrow separately provided for within the US GAAP taxonomy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.10) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition Lease area in which a working interest is owned and on which wells have not been drilled or completed to a point that would permit the production of commercial quantities of oil and gas regardless of whether or not such area contains proved reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Industry Guide -Number 2 -Paragraph 5 + Details
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- Definition Fractional interest owned based on working interests or other economic arrangements of lease area on which wells have not been drilled or completed to a point that would permit the production of commercial quantities of oil and gas regardless of whether or not such area contains proved reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Industry Guide -Number 2 -Paragraph 5 + Details
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- Definition The cash outflow to purchase long lived physical asset for use in the normal oil and gas operations and to purchase mineral interests in oil and gas properties not intended for resale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3213-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 17 -Subparagraph c -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 15 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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