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Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | $ 4,473.2 | $ 5,053.4 | $ 4,991.1
Other revenues | 208.5 | 155.5 | 123.4
Total Operating Revenues | 4,681.7 | 5,208.9 | 5,114.5
Gas Distribution Operations |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 3,032.0 | 3,455.2 | 3,372.0
Other revenues | 96.1 | 54.5 | 34.4
Total Operating Revenues | 3,128.1 | 3,509.7 | 3,406.4
Electric Operations |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 1,440.4 | 1,597.4 | 1,618.4
Other revenues | 95.5 | 101.0 | 89.0
Total Operating Revenues | 1,535.9 | 1,698.4 | 1,707.4
Corporate and Other |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 0.8 | 0.8 | 0.7
Other revenues | 16.9 | 0.0 | 0.0
Total Operating Revenues | 17.7 | 0.8 | 0.7
Residential |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 2,602.8 | 2,790.6 | 2,744.7
Residential | Gas Distribution Operations |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 2,075.0 | 2,309.0 | 2,250.0
Residential | Electric Operations |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 527.8 | 481.6 | 494.7
Residential | Corporate and Other |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 0.0 | 0.0 | 0.0
Commercial |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 1,150.8 | 1,257.9 | 1,244.6
Commercial | Gas Distribution Operations |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 670.5 | 771.3 | 751.9
Commercial | Electric Operations |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 480.3 | 486.6 | 492.7
Commercial | Corporate and Other |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 0.0 | 0.0 | 0.0
Industrial |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 624.9 | 852.9 | 841.6
Industrial | Gas Distribution Operations |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 212.8 | 245.2 | 228.0
Industrial | Electric Operations |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 412.1 | 607.7 | 613.6
Industrial | Corporate and Other |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 0.0 | 0.0 | 0.0
Off-system |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 41.0 | 77.7 | 92.4
Off-system | Gas Distribution Operations |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 41.0 | 77.7 | 92.4
Off-system | Electric Operations |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 0.0 | 0.0 | 0.0
Off-system | Corporate and Other |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 0.0 | 0.0 | 0.0
Miscellaneous |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 53.7 | 74.3 | 67.8
Miscellaneous | Gas Distribution Operations |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 32.7 | 52.0 | 49.7
Miscellaneous | Electric Operations |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | 20.2 | 21.5 | 17.4
Miscellaneous | Corporate and Other |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Customer revenues | $ 0.8 | $ 0.8 | $ 0.7

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