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Operations and Significant Accounting Policies - New Accounting Pronouncements (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2017 | Jun. 30, 2017 | Jun. 30, 2018 | Dec. 31, 2017
New Accounting Pronouncements [Line Items] |  |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | [1] |  |  |  | $ 0.5
Operating Leases, Future Minimum Payments Due |  |  |  | $ 80.0 |
Accounting Standards Update 2018-02 [Member] |  |  |  |  |
New Accounting Pronouncements [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 Pronouncements [Line Items] |  |  |  |  |
Prior Period Reclassification Amount |  | $ 1.0 | $ 2.1 |  |
Accounting Standards Update 2017-07 [Member] | Cost of Sales - Non-utility [Member] |  |  |  |  |
New Accounting Pronouncements [Line Items] |  |  |  |  |
Prior Period Reclassification Amount |  |  | 0.1 |  |
Accounting Standards Update 2017-07 [Member] | Other Income (Expense) [Member] |  |  |  |  |
New Accounting Pronouncements [Line Items] |  |  |  |  |
Prior Period Reclassification Amount |  | $ 1.0 | 2.0 |  |
Accounting Standards Update 2016-01 [Member] |  |  |  |  |
New Accounting Pronouncements [Line Items] |  |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption |  |  |  |  | 0.1
Accounting Standards Update 2016-18 [Member] |  |  |  |  |
New Accounting Pronouncements [Line Items] |  |  |  |  |
Prior Period Reclassification Amount |  |  | $ 1.5 |  |
Accounting Standards Update 2014-09 [Member] |  |  |  |  |
New Accounting Pronouncements [Line Items] |  |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption |  |  |  |  | $ 0.5
[1] | Reflects the impacts associated with the adoption of accounting standards concerning Financial Instruments, Revenue from Contracts with Customers and the Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. (See Note 1. Operations and Significant Accounting Policies.)

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- Definition Amount of required minimum rental payments for leases having an initial or remaining non-cancelable letter-terms in excess of one year. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 10 -Section 55 -Paragraph 40 -Subparagraph (Note 3) -URI http://asc.fasb.org/extlink&oid=82846649&loc=d3e38371-112697 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41502-112717 + Details
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