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Property, Plant and Equipment - Additional Information (Detail) (USD $) | 12 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2010 Roosevelt County, Montana [Member] acre | Dec. 31, 2010 Richland County, Montana [Member] acre | Dec. 31, 2012 Unproved property leases [Member] | Dec. 31, 2011 Unproved property leases [Member] | Dec. 31, 2010 Unproved property leases [Member]
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |
Asset retirement costs | $ 20,700,000 | $ 11,400,000 |  |  |  |  |  |
Impairment of oil and gas properties | 3,581,000 | 3,610,000 | 11,967,000 |  |  | 3,600,000 | 3,600,000 | 12,000,000
Number of acres acquired |  |  |  | 16,700 | 10,000 |  |  |
Payments to acquire land |  |  |  | $ 52,300,000 | $ 30,100,000 |  |  |

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- Definition The amount of asset retirement costs previously incurred and capitalized separately from the capitalized amount of the associated long-lived assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61901-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 19 -Paragraph 59M -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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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- Definition The expense recorded to reduce the value of oil and gas assets consisting of proved properties and unproved properties as the estimate of future successful production from these properties is reduced. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 19 -Paragraph 27, 28, 29 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 360 -Section 35 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=6475018&loc=d3e66150-109466 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6391110&loc=d3e2921-110230 + Details
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- Definition The cash outflow from the acquisition of real estate intended to generate income for the owner; excludes land acquired for use by the owner. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 17 -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 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 + Details
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