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Regulatory Assets (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Regulatory Assets [Line Items] |  |  |
Regulatory Assets |  | $ 149.1 | $ 142.0
Less: Current Portion of Regulatory Assets | [1] | 39.5 | 37.9
Regulatory Assets-noncurrent |  | 109.6 | 104.1
Environmental Matters |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets |  | 9.5 | 10.8
Other Assets |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets |  | 5.4 | 5.7
Retirement Benefits |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets |  | 84.5 | 75.9
Deferred Storm Charges |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets |  | 7.2 | 9.6
Income Taxes |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets |  | 6.5 | 7.3
Energy Supply & Other Rate Adjustment Mechanisms |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets |  | $ 36.0 | $ 32.7
[1] | Reflects amounts included in Accrued Revenue on the Company's Consolidated Balance Sheets and in the Accrued Revenue table shown above.

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources 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 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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