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Fair Value Amount of Derivative Assets Liabilities Offset in Regulatory Liabilities Assets (Detail) (Not Designated as Hedging Instrument, USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013
Derivative Assets
Derivative Assets	$ 0.2	$ 0.2	$ 0.5
Derivative Liabilities
Derivative Liabilities
Natural Gas Future/Options Contract \| Prepayments and Other
Derivative Assets
Derivative Assets	0.2	0.1	0.5
Natural Gas Future/Options Contract \| Other Assets
Derivative Assets
Derivative Assets		0.1
Natural Gas Future/Options Contract \| Other Current Liabilities
Derivative Liabilities
Derivative Liabilities
Natural Gas Future/Options Contract \| Other Noncurrent Liabilities
Derivative Liabilities
Derivative Liabilities

Fair value of financial asset or other 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 an obligation to return collateral under a master netting arrangement.

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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+ Details