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Recent Accounting Pronouncements (Schedule of New Accounting Pronouncements and Changes in Accounting Principles) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
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Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Operation and maintenance |  |  |  |  |  |  |  |  | $ 2,352.9 | $ 1,601.7 |  | $ 1,445.8 |
Total Operating Expenses |  |  |  |  |  |  |  |  | 4,989.8 | 3,953.4 |  | 3,626.4 |
Operating Income | $ (78.4) | $ (315.9) | $ 118.4 | $ 400.6 | $ 270.6 | $ 111.2 | $ 124.0 | $ 415.4 | 124.7 | 921.2 |  | 866.1 |
Other Income (Deductions) |  |  |  |  |  |  |  |  |  |  |  |  |
Other, net |  |  |  |  |  |  |  |  | 43.5 | (13.5) |  | (3.0) |
Total Other Deductions |  |  |  |  |  |  |  |  | (355.3) | (478.2) |  | (352.5) |
Previously Reported [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Operation and maintenance |  |  |  |  |  |  |  |  |  | 1,612.3 |  | 1,453.7 |
Total Operating Expenses |  |  |  |  |  |  |  |  |  | 3,964.0 |  | 3,634.3 |
Operating Income |  |  |  |  |  |  |  |  |  | 910.6 |  | 858.2 |
Other Income (Deductions) |  |  |  |  |  |  |  |  |  |  |  |  |
Other, net |  |  |  |  |  |  |  |  |  | (2.8) |  | 1.5 |
Total Other Deductions |  |  |  |  |  |  |  |  |  | (467.5) |  | (348.0) |
Restatement Adjustment [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Operation and maintenance |  |  |  |  |  |  |  |  |  | 1,601.7 |  | 1,445.8 |
Total Operating Expenses |  |  |  |  |  |  |  |  |  | 3,953.4 |  | 3,626.4 |
Operating Income |  |  |  |  |  |  |  |  |  | 921.2 |  | 866.1 |
Other Income (Deductions) |  |  |  |  |  |  |  |  |  |  |  |  |
Other, net |  |  |  |  |  |  |  |  |  | (13.4) |  | (6.4) |
Total Other Deductions |  |  |  |  |  |  |  |  |  | (478.1) |  | (355.9) |
Accounting Standards Update 2017-07 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Operation and maintenance |  |  |  |  |  |  |  |  |  | (10.6) | [1] | (7.9) | [2]
Total Operating Expenses |  |  |  |  |  |  |  |  |  | (10.6) | [1] | (7.9) | [2]
Operating Income |  |  |  |  |  |  |  |  |  | 10.6 | [1] | 7.9 | [2]
Other Income (Deductions) |  |  |  |  |  |  |  |  |  |  |  |  |
Other, net |  |  |  |  |  |  |  |  |  | (10.6) | [1] | (7.9) | [2]
Total Other Deductions |  |  |  |  |  |  |  |  |  | (10.6) | [1] | (7.9) | [2]
Gas Distribution Operations |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Operating Income |  |  |  |  |  |  |  |  | (254.1) | 550.1 |  | 569.7 |
Other Income (Deductions) |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Operating Results |  |  |  |  |  |  |  |  |  | (4.4) |  | 4.3 |
Electric Operations |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Operating Income |  |  |  |  |  |  |  |  | 386.1 | 367.4 |  | 301.3 |
Other Income (Deductions) |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Operating Results |  |  |  |  |  |  |  |  |  | (2.6) |  | (9.8) |
Corporate and Other |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Operating Income |  |  |  |  |  |  |  |  | $ (7.3) | 3.7 |  | (4.9) |
Other Income (Deductions) |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Operating Results |  |  |  |  |  |  |  |  |  | $ (3.6) |  | $ (2.4) |
[1] | The effect of this change is attributable to our business segments: Gas Distribution Operations, Electric Operations, and Corporate and Other in the amounts of $(4.4) million, $(2.6) million, and $(3.6) million, respectively.
[2] | The effect of this change is attributable to our business segments: Gas Distribution Operations, Electric Operations, and Corporate and Other in the amounts of $4.3 million, $(9.8) million, and $(2.4) million, respectively.

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