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Fair Value Amount of Derivative Liabilities Offset in Regulatory Assets (Detail) (Not Designated as Hedging Instrument, USD $) In Millions, unless otherwise specified | Jun. 30, 2013 | Dec. 31, 2012 | Jun. 30, 2012
--- | --- | --- | ---
Derivative Liabilities |  |  |
Natural Gas Futures Contracts | $ 0.4 | $ 0.7 | $ 1.5
Natural Gas Contracts | Other Current Liabilities |  |  |
Derivative Liabilities |  |  |
Natural Gas Futures Contracts | 0.3 | 0.7 | 1.5
Natural Gas Contracts | Other Noncurrent Liabilities |  |  |
Derivative Liabilities |  |  |
Natural Gas Futures Contracts | $ 0.1 |  |

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- Definition Fair value of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be offset against a right to receive collateral under a master netting arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=20225523&loc=SL20225862-175312 Reference 2: 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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=28370219&loc=SL20226011-175313 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=28370219&loc=SL20226006-175313 + Details
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