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Utility Regulatory Assets and Liabilities and Regulatory Matters - Regulatory Assets and Liabilities Associated with Gas Utility and Electric Utility (Details) - USD ($)
$ in Millions
Dec. 31, 2015
Sep. 30, 2015
Dec. 31, 2014
Regulatory Assets And Liabilities      
Regulatory assets $ 301.8 $ 304.2 $ 270.6
Regulatory liabilities [1] 63.2 71.0 32.9
Postretirement Benefits      
Regulatory Assets And Liabilities      
Regulatory liabilities [1] 20.3 20.0 19.0
Deferred Fuel and Power Refunds      
Regulatory Assets And Liabilities      
Regulatory liabilities [1] 28.1 36.6 0.0
State Tax Benefits—Distribution System Repairs      
Regulatory Assets And Liabilities      
Regulatory liabilities [1] 13.7 13.3 10.3
Other      
Regulatory Assets And Liabilities      
Regulatory liabilities [1] 1.1 1.1 3.6
Income Taxes Recoverable      
Regulatory Assets And Liabilities      
Regulatory assets 117.4 115.9 111.1
Underfunded Pension and Postretirement Plans      
Regulatory Assets And Liabilities      
Regulatory assets 138.3 140.8 107.8
Environmental Costs      
Regulatory Assets And Liabilities      
Regulatory assets 17.6 20.0 14.7
Deferred Fuel and Power Costs      
Regulatory Assets And Liabilities      
Regulatory assets 0.0 0.0 16.7
Removal Costs, Net      
Regulatory Assets And Liabilities      
Regulatory assets 22.3 21.2 17.6
Other      
Regulatory Assets And Liabilities      
Regulatory assets $ 6.2 $ 6.3 $ 2.7
[1] Regulatory liabilities are recorded in other current and other noncurrent liabilities in the Condensed Consolidated Balance Sheets.