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Report of Business Segments (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Segment Reporting Information [Line Items]      
Operating revenues $ 492.7 $ 456.0  
Cost of sales 210.5 208.7  
Other operation and maintenance 118.8 122.1  
Depreciation and amortization 78.8 55.6  
Taxes other than income 24.1 23.9  
OPERATING INCOME 60.5 45.7  
Equity in earnings of unconsolidated affiliates 33.9 35.6  
Pension and Other Postretirement Benefits Cost (Reversal of Cost) (1.3) 1.9  
Other income (expense) 9.3 9.7  
Interest expense 38.6 35.0  
Income tax expense (benefit) 10.1 20.0  
Net income 55.0 36.0  
Investment in unconsolidated affiliates 1,160.6 1,158.9 $ 1,160.4
Total assets 10,436.4 10,134.9 $ 10,412.7
Electric Utility [Member]      
Segment Reporting Information [Line Items]      
Operating revenues 492.7 456.0  
Cost of sales 210.5 208.7  
Other operation and maintenance 119.7 124.7  
Depreciation and amortization 78.8 54.7  
Taxes other than income 22.7 22.3  
OPERATING INCOME 61.0 45.6  
Equity in earnings of unconsolidated affiliates 0.0 0.0  
Other income (expense) 10.6 11.5  
Interest expense 37.3 33.6  
Income tax expense (benefit) 3.0 7.3  
Net income 31.3 16.2  
Investment in unconsolidated affiliates 0.0 0.0  
Total assets 9,303.3 8,950.9  
Natural Gas Midstream Operations [Member]      
Segment Reporting Information [Line Items]      
Operating revenues 0.0 0.0  
Cost of sales 0.0 0.0  
Other operation and maintenance 0.3 0.1  
Depreciation and amortization 0.0 0.0  
Taxes other than income 0.2 0.2  
OPERATING INCOME (0.5) (0.3)  
Equity in earnings of unconsolidated affiliates 33.9 35.6  
Other income (expense) 0.0 0.1  
Interest expense 0.0 0.0  
Income tax expense (benefit) 9.8 15.4  
Net income 23.6 20.0  
Investment in unconsolidated affiliates 1,150.6 1,158.9  
Total assets 1,159.0 1,521.2  
Other Operations [Member]      
Segment Reporting Information [Line Items]      
Operating revenues 0.0 0.0  
Cost of sales 0.0 0.0  
Other operation and maintenance (1.2) (2.7)  
Depreciation and amortization 0.0 0.9  
Taxes other than income 1.2 1.4  
OPERATING INCOME 0.0 0.4  
Equity in earnings of unconsolidated affiliates 0.0 0.0  
Other income (expense) (0.7) (1.8)  
Interest expense 1.9 1.5  
Income tax expense (benefit) (2.7) (2.7)  
Net income 0.1 (0.2)  
Investment in unconsolidated affiliates 10.0 0.0  
Total assets 98.3 91.1  
Eliminations [Member]      
Segment Reporting Information [Line Items]      
Operating revenues 0.0 0.0  
Cost of sales 0.0 0.0  
Other operation and maintenance 0.0 0.0  
Depreciation and amortization 0.0 0.0  
Taxes other than income 0.0 0.0  
OPERATING INCOME 0.0 0.0  
Equity in earnings of unconsolidated affiliates 0.0 0.0  
Other income (expense) (0.6) (0.1)  
Interest expense (0.6) (0.1)  
Income tax expense (benefit) 0.0 0.0  
Net income 0.0 0.0  
Investment in unconsolidated affiliates 0.0 0.0  
Total assets $ (124.2) $ (428.3)