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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

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.

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