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Operations and Significant Accounting Policies - Reclassification of Prior Income Statement (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Operating Revenue – Utility [Member] |  |
Schedule of Prior Period Reclassification Adjustments [Line Items] |  |
Prior Period Reclassification Adjustment | $ 1,063.8 | $ 1,000.7
Operating Revenue – Non-utility [Member] |  |
Schedule of Prior Period Reclassification Adjustments [Line Items] |  |
Prior Period Reclassification Adjustment | 331.9 | 316.7
Other Operating Revenue - Non-utility [Member] |  |
Schedule of Prior Period Reclassification Adjustments [Line Items] |  |
Prior Period Reclassification Adjustment | $ 23.6 | $ 22.3

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- Definition [Line Items] for Schedule of prior period reclassification adjustments to conform the presentation of prior period financial statements with the presentation in the current period. + References No definition available. + Details
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- Definition The amount of a reclassification adjustment made to prior period financial statement amounts. + References No definition available. + Details
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- Details
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- Details
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- Details
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