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Natural gas costs recoverable or refundable through rate adjustments (Details 8) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Accounting Policies [Abstract] |  |
Natural gas costs refundable | $ 28.5 | $ 25.6
Natural gas costs recoverable | $ 14.5 | $ 2.2

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- 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=77907324&loc=d3e43603-110378 + Details
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- 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 980 -SubTopic 405 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 Reference 2: 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 + Details
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