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Alaska Production Credits (Details Textual) (USD $)
In Thousands, unless otherwise specified
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3 Months Ended |
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Jul. 31, 2014
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Jul. 31, 2013
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Apr. 30, 2014
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| Alaska Production Tax Credits [Line Items] |
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| Net carried-forward annual loss credits |
$ 3,055 |
[1] |
$ 0 |
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| Expenditure and exploration credits |
70,025 |
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| Alaska production credits receivable, net |
$ 53,614 |
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|
$ 49,121 |
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43.55.023(a)(1) [Member]
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| Alaska Production Tax Credits [Line Items] |
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| Alaska capital expenditure credit (percent) |
20.00% |
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43.55.023(1)(1) [Member]
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| Alaska Production Tax Credits [Line Items] |
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| Alaska capital expenditure credit (percent) |
40.00% |
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43.55.023(a)(2) [Member]
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| Alaska Production Tax Credits [Line Items] |
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| Alaska capital exploration credit (percent) |
20.00% |
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43.55.023(1)(2) [Member]
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| Alaska Production Tax Credits [Line Items] |
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| Alaska capital exploration credit (percent) |
40.00% |
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43.55.023(b) [Member]
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| Alaska Production Tax Credits [Line Items] |
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| Alaska carried-forward annual loss credit (percent) |
25.00% |
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43.55.025 [Member]
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| Alaska Production Tax Credits [Line Items] |
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| Seismic exploration credits (percent) |
40.00% |
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