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Operations and Significant Accounting Policies - New Accounting Pronouncements (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2018
New Accounting Standards [Line Items] |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | $ 0.5 |  |
Accounting Standards Update 2018-02 [Member] |  |  |
New Accounting Standards [Line Items] |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption |  |  | $ 5.7
Accounting Standards Update 2017-07 [Member] | Operating and Maintenance [Member] |  |  |
New Accounting Standards [Line Items] |  |  |
Prior Period Reclassification Adjustment | 4.2 | $ 6.8 |
Accounting Standards Update 2017-07 [Member] | Cost of Sales - Non-utility [Member] |  |  |
New Accounting Standards [Line Items] |  |  |
Prior Period Reclassification Adjustment | 0.3 |  |
Accounting Standards Update 2017-07 [Member] | Other Income/Expense [Member] |  |  |
New Accounting Standards [Line Items] |  |  |
Prior Period Reclassification Adjustment | 3.9 | 6.5 |
Accounting Standards Update 2016-01 [Member] |  |  |
New Accounting Standards [Line Items] |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | 0.1 |  |
Accounting Standards Update 2016-15 [Member] |  |  |
New Accounting Standards [Line Items] |  |  |
Prior Period Reclassification Adjustment |  | 2.9 |
Accounting Standards Update 2014-09 [Member] |  |  |
New Accounting Standards [Line Items] |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | 0.5 |  |
Accounting Standards Update 2016-02 [Member] |  |  |
New Accounting Standards [Line Items] |  |  |
Expected Minimum Lease Payment Due in Future Years |  |  | $ 36.0
Financing Activities [Member] | Accounting Standards Update 2016-18 [Member] |  |  |
New Accounting Standards [Line Items] |  |  |
Prior Period Reclassification Adjustment | $ 0.4 | 0.7 |
Investing Activities [Member] | Accounting Standards Update 2016-18 [Member] |  |  |
New Accounting Standards [Line Items] |  |  |
Prior Period Reclassification Adjustment |  | $ 4.1 |

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