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Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
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Regulatory Assets [Line Items] |  |
Total Assets | $ 2,237.5 | $ 1,801.2
Unrecognized Pension Benefit And Other Postretirement Benefit Costs |  |
Regulatory Assets [Line Items] |  |
Total Assets | 798.3 | 733.5
Other Postretirement Costs |  |
Regulatory Assets [Line Items] |  |
Total Assets | 74.1 | 70.7
Environmental Costs |  |
Regulatory Assets [Line Items] |  |
Total Assets | 61.5 | 63.4
Regulatory Effects Of Accounting For Income Taxes |  |
Regulatory Assets [Line Items] |  |
Total Assets | 233.1 | 238.8
Underrecovered Gas And Fuel Costs |  |
Regulatory Assets [Line Items] |  |
Total Assets | 34.7 | 25.5
Depreciation |  |
Regulatory Assets [Line Items] |  |
Total Assets | 209.6 | 181.0
Post-In Service Carrying Charges |  |
Regulatory Assets [Line Items] |  |
Total Assets | 206.6 | 173.3
Safety Activity Costs |  |
Regulatory Assets [Line Items] |  |
Total Assets | 91.7 | 66.5
DSM Program |  |
Regulatory Assets [Line Items] |  |
Total Assets | 45.5 | 40.0
Bailly Generating Station [Member] |  |
Regulatory Assets [Line Items] |  |
Total Assets | 244.3 | 0.0
Other Assets |  |
Regulatory Assets [Line Items] |  |
Total Assets | $ 238.1 | $ 208.5

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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://fasb.org/us-gaap/role/ref/legacyRef -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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