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Natural gas costs recoverable or refundable through rate adjustments (Details 10) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Accounting Policies [Abstract]
Minimum period over which natural gas costs greater or less than amounts being recovered become recoverable or refundable through rate adjustments	12 months
Maximum period over which natural gas costs greater or less than amounts being recovered become recoverable or refundable through rate adjustments	28 months
Natural gas costs refundable	$ 13.2us-gaap_RefundableGasCosts	$ 16.9us-gaap_RefundableGasCosts
Natural gas costs recoverable	$ 19.6us-gaap_DeferredGasCost	$ 12.1us-gaap_DeferredGasCost

X

- Definition

Maximum period over which natural gas costs greater or less than amounts being recovered through existing rate schedules become recoverable or refundable through rate adjustments.

+ References

No definition available.

+ Details

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X

- Definition

Minimum period over which natural gas costs greater or less than amounts being recovered through existing rate schedules become recoverable or refundable through rate adjustments.

+ References

No definition available.

+ Details

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- Details

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Namespace Prefix:	us-gaap_
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X

- Definition

Carrying amount as of the balance sheet date of capitalized gas costs of a regulated entity that are expected to be recoverable through rate adjustments within one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 340

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6499739&loc=d3e43603-110378

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Gas costs of a regulated entity that are refundable through future rate adjustments. Such costs are a form of a regulatory liabilities and are expected to be refunded in less than one year through rate adjustments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.20)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 405

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394

+ Details

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Balance Type:	credit
Period Type:	instant