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Acquisitions (Details) (USD $)
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0 Months Ended | 1 Months Ended | 3 Months Ended | 12 Months Ended | 0 Months Ended | 23 Months Ended | 0 Months Ended | ||||||||||||||||
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Dec. 24, 2012
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Feb. 28, 2011
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Mar. 31, 2013
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Mar. 31, 2012
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Dec. 31, 2012
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Dec. 31, 2011
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Dec. 31, 2010
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Feb. 15, 2013
Utica Shale [Member]
well
acre
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Dec. 21, 2012
Utica Shale [Member]
acre
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Dec. 18, 2012
Utica Shale [Member]
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Dec. 19, 2012
Utica Shale [Member]
acre
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Dec. 24, 2012
Utica Shale [Member]
acre
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Dec. 31, 2012
Utica Shale [Member]
acre
wells
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Dec. 17, 2012
Utica Shale [Member]
acre
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Feb. 15, 2013
Common Stock [Member]
Public Offering [Member]
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Jan. 07, 2013
Common Stock [Member]
Public Offering [Member]
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Dec. 24, 2012
Common Stock [Member]
Public Offering [Member]
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Jan. 07, 2013
Common Stock [Member]
Public Offering [Member]
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Dec. 05, 2011
Common Stock [Member]
Public Offering [Member]
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Jul. 15, 2011
Common Stock [Member]
Public Offering [Member]
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Mar. 30, 2011
Common Stock [Member]
Public Offering [Member]
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May 25, 2010
Common Stock [Member]
Public Offering [Member]
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May 25, 2010
Common Stock [Member]
Public Offering [Member]
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| 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 | ||||||||||||||||||||||