Fair Value Amount of Derivative Assets Liabilities Offset in Regulatory Liabilities Assets (Detail) (Not Designated as Hedging Instrument, USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivative Assets
Derivative Assets	$ 0.1	$ 0.2
Derivative Liabilities
Derivative Liabilities	0	0
Natural Gas Future/Options Contract \| Prepayments and Other
Derivative Assets
Derivative Assets		0.1
Natural Gas Future/Options Contract \| Other Noncurrent Assets
Derivative Assets
Derivative Assets	0.1	0.1
Natural Gas Future/Options Contract \| Other Current Liabilities
Derivative Liabilities
Derivative Liabilities	0	0
Natural Gas Future/Options Contract \| Other Noncurrent Liabilities
Derivative Liabilities
Derivative Liabilities	$ 0	$ 0

Fair value, after offset of derivative liability, 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, before offset against an obligation to return collateral under a master netting arrangement. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, after offset of derivative asset, 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 and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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