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Fair Value Amount of Derivative Assets and Liabilities Offset in Regulatory Liabilities and Assets (Detail) (Not Designated as Hedging Instrument, USD $) In Millions, unless otherwise specified | Mar. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2012
--- | --- | --- | ---
Derivative Assets |  |  |
Natural Gas Futures Contracts | $ 0.5 |  |
Derivative Liabilities |  |  |
Natural Gas Futures Contracts |  | 0.7 | 2.0
Natural Gas Contracts | Prepayments and other |  |  |
Derivative Assets |  |  |
Natural Gas Futures Contracts | 0.5 |  |
Natural Gas Contracts | Other Current Liabilities |  |  |
Derivative Liabilities |  |  |
Natural Gas Futures Contracts |  | 0.7 | 1.8
Natural Gas Contracts | Other Noncurrent Liabilities |  |  |
Derivative Liabilities |  |  |
Natural Gas Futures Contracts |  |  | $ 0.2

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- Definition The amount as of the balance sheet date of the fair value of derivative assets that in accordance with the entity's accounting policy was not offset against an obligation to return cash collateral under a master netting arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=7476318&loc=d3e41678-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FIN39-1 -Paragraph 10B -Subparagraph a -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 amount as of the balance sheet date of the fair value of derivative liabilities that in accordance with the entity's accounting policy was not offset against the right to reclaim cash collateral under a master netting arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=7476318&loc=d3e41678-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FIN39-1 -Paragraph 10B -Subparagraph a -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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