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.

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

Name:	us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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