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Report of Business Segments (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Segment Reporting Information [Line Items]          
Operating revenues $ 567.0 $ 586.4 $ 1,059.7 $ 1,042.4  
Cost of sales 208.7 232.1 419.2 440.8  
Other operation and maintenance 123.2 112.5 242.0 234.6  
Depreciation and amortization 80.9 74.7 159.7 130.3  
Taxes other than income 22.5 21.3 46.6 45.2  
OPERATING INCOME 131.7 145.8 192.2 191.5  
Equity in earnings of unconsolidated affiliates 29.3 29.4 63.2 65.0  
Other income (expense) 8.5 13.3 17.8 23.0  
Interest expense 40.9 37.1 79.5 72.1  
Income tax expense 17.9 46.6 28.0 66.6  
Net income (loss) 110.7 104.8 165.7 140.8  
Investment in unconsolidated affiliates 1,153.6 1,159.1 1,153.6 1,159.1 $ 1,160.4
Total assets 10,543.6 10,420.5 10,543.6 10,420.5 $ 10,412.7
Electric Utility [Member]          
Segment Reporting Information [Line Items]          
Operating revenues 567.0 586.4 1,059.7 1,042.4  
Cost of sales 208.7 232.1 419.2 440.8  
Other operation and maintenance 123.5 114.7 243.2 239.4  
Depreciation and amortization 80.9 73.7 159.7 128.4  
Taxes other than income 21.6 20.2 44.3 42.5  
OPERATING INCOME 132.3 145.7 193.3 191.3  
Equity in earnings of unconsolidated affiliates 0.0 0.0 0.0 0.0  
Other income (expense) 10.0 13.8 20.6 25.3  
Interest expense 39.2 35.6 76.5 69.2  
Income tax expense 11.1 37.7 14.1 45.0  
Net income (loss) 92.0 86.2 123.3 102.4  
Investment in unconsolidated affiliates 0.0 0.0 0.0 0.0  
Total assets 9,447.8 9,199.0 9,447.8 9,199.0  
Natural Gas Midstream Operations [Member]          
Segment Reporting Information [Line Items]          
Operating revenues 0.0 0.0 0.0 0.0  
Cost of sales 0.0 0.0 0.0 0.0  
Other operation and maintenance 0.3 0.2 0.6 0.3  
Depreciation and amortization 0.0 0.0 0.0 0.0  
Taxes other than income 0.2 0.3 0.4 0.5  
OPERATING INCOME (0.5) (0.5) (1.0) (0.8)  
Equity in earnings of unconsolidated affiliates 29.3 29.4 63.2 65.0  
Other income (expense) 0.0 0.0 0.0 0.1  
Interest expense 0.0 0.0 0.0 0.0  
Income tax expense 6.7 10.6 16.5 26.0  
Net income (loss) 22.1 18.3 45.7 38.3  
Investment in unconsolidated affiliates 1,144.6 1,153.9 1,144.6 1,153.9  
Total assets 1,147.9 1,513.9 1,147.9 1,513.9  
Other Operations [Member]          
Segment Reporting Information [Line Items]          
Operating revenues 0.0 0.0 0.0 0.0  
Cost of sales 0.0 0.0 0.0 0.0  
Other operation and maintenance (0.6) (2.4) (1.8) (5.1)  
Depreciation and amortization 0.0 1.0 0.0 1.9  
Taxes other than income 0.7 0.8 1.9 2.2  
OPERATING INCOME (0.1) 0.6 (0.1) 1.0  
Equity in earnings of unconsolidated affiliates 0.0 0.0 0.0 0.0  
Other income (expense) (0.6) (0.5) (1.3) (2.3)  
Interest expense 2.6 1.5 4.5 3.0  
Income tax expense 0.1 (1.7) (2.6) (4.4)  
Net income (loss) (3.4) 0.3 (3.3) 0.1  
Investment in unconsolidated affiliates 9.0 5.2 9.0 5.2  
Total assets 95.6 89.2 95.6 89.2  
Eliminations [Member]          
Segment Reporting Information [Line Items]          
Operating revenues 0.0 0.0 0.0 0.0  
Cost of sales 0.0 0.0 0.0 0.0  
Other operation and maintenance 0.0 0.0 0.0 0.0  
Depreciation and amortization 0.0 0.0 0.0 0.0  
Taxes other than income 0.0 0.0 0.0 0.0  
OPERATING INCOME 0.0 0.0 0.0 0.0  
Equity in earnings of unconsolidated affiliates 0.0 0.0 0.0 0.0  
Other income (expense) (0.9) 0.0 (1.5) (0.1)  
Interest expense (0.9) 0.0 (1.5) (0.1)  
Income tax expense 0.0 0.0 0.0 0.0  
Net income (loss) 0.0 0.0 0.0 0.0  
Investment in unconsolidated affiliates 0.0 0.0 0.0 0.0  
Total assets $ (147.7) $ (381.6) $ (147.7) $ (381.6)