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Subsequent Events (Details) (USD $) In Thousands, except Share data, unless otherwise specified | 0 Months Ended | 3 Months Ended | 12 Months Ended | 0 Months Ended |  |  |  |  | 0 Months Ended | 1 Months Ended |  |  | 0 Months Ended
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Dec. 24, 2012 | Mar. 31, 2013 | Mar. 31, 2012 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Feb. 15, 2013 Utica Shale [Member] well acre | Dec. 24, 2012 Utica Shale [Member] acre | Dec. 31, 2012 Utica Shale [Member] acre | Dec. 21, 2012 Utica Shale [Member] acre | Dec. 19, 2012 Utica Shale [Member] acre | Dec. 17, 2012 Utica Shale [Member] acre | Feb. 15, 2013 Subsequent Event [Member] Utica Shale [Member] well acre | Feb. 28, 2013 Subsequent Event [Member] Utica Shale [Member] | Jan. 07, 2013 Subsequent Event [Member] Utica Shale [Member] acre | Dec. 31, 2012 Subsequent Event [Member] Utica Shale [Member] | 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] | May 19, 2010 Common Stock [Member] Public Offering [Member] | Feb. 15, 2013 Common Stock [Member] Public Offering [Member] Subsequent Event [Member] | Jan. 07, 2013 Common Stock [Member] Public Offering [Member] Subsequent Event [Member] | Jan. 07, 2013 Common Stock [Member] Public Offering [Member] Subsequent Event [Member]
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock in public offering, shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8,912,500 |  | 11,750,000 |  | 4,600,000 | 3,450,000 | 2,760,000 |  |  | 1,481,481 | 8,912,500 |  |
Stock option, portion attributable to underwriters (shares) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 900,000 | 750,000 |  |  |  |  | 187,022 |  |  |  |  |
Stock option, portion available to underwriters (shares) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,650,000 |  |  |  |  |  |  |  |  | 900,000 |
Proceeds from issuance of common stock | $ 427,900 | $ 357,640 |  | $ 427,091 | $ 307,154 | $ 21,879 | $ 325,800 | $ 460,700 |  |  |  |  |  |  |  |  | $ 325,800 |  | $ 427,900 | $ 460,700 | $ 128,000 | $ 94,700 | $ 84,300 |  | $ 21,600 |  | $ 325,800 |  | $ 460,700
Share price (usd per share) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 38.00 |  | $ 38.00 |  | $ 29.00 | $ 28.75 | $ 32.00 |  |  | $ 13.50 | $ 38.00 |  |
Gas and oil areage, undeveloped, net |  |  |  |  |  |  | 22,000 | 37,000 | 106,000 | 37,000 | 7,000 | 30,000 | 22,000 |  | 37,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash price of ownership interest |  | $ 320,489 | $ 84,778 | $ 757,192 | $ 287,292 | $ 101,644 |  |  |  |  |  |  | $ 220,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of wells excluded from acquisition |  |  |  |  |  |  |  |  |  |  |  |  | 14 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of proposed future wells excluded from acquisition |  |  |  |  |  |  | 16 |  |  |  |  |  | 16 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage of acreage |  |  |  |  |  |  |  |  |  |  |  |  |  | 93.80% |  | 77.70% |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage, additional acreage purchased |  |  |  |  |  |  |  |  |  |  |  |  |  | 16.20% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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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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