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Operations and Significant Accounting Policies - New Accounting Pronouncements (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2017
Mar. 31, 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 $ 1.1    
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    
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 $ 6.9    
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 recently adopted 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.)