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Pension and Other Postretirement Benefits (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |  |  |  | $ 2,084.1 |  | $ 2,422.5 |  |
Regulatory assets |  |  |  |  |  | 2,237.5 |  | 1,801.2 |  |
Regulatory Liabilities |  |  |  |  |  | $ 2,660.0 |  | 2,795.6 |  |
Expected return on plan assets |  |  |  |  |  | 7.00% |  |  |  |
Pension Plan |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |  |  |  | $ 1,867.7 |  | 2,160.0 |  | $ 1,750.9
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) |  |  |  |  |  | (127.5) |  | 76.1 |  |
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) |  | $ (0.4) | $ (0.7) |  |  | (0.4) |  | (0.7) |  | (0.2)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) |  |  |  |  |  | 14.8 |  | 41.1 |  | $ 48.5
Employer contributions |  |  |  | $ 277.0 |  | 2.9 |  | 282.3 |  |
Expected contribution |  |  |  |  |  | 3.0 |  |  |  |
Defined Benefit Plan, Accumulated Benefit Obligation |  |  |  |  |  | 1,965.6 |  | 2,170.4 |  |
Funded status of plan |  |  |  |  |  | 113.6 |  | 32.6 |  |
Liability, Defined Benefit Plan | [1] |  |  |  |  | $ 113.6 |  | $ 32.6 |  |
Expected return on plan assets |  |  |  |  |  | 7.00% |  | 7.25% |  | 8.00%
Settlement loss |  | 18.5 | 13.7 |  |  | $ 18.5 |  | $ 13.7 |  | $ 0.0
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement |  |  |  |  |  | 3.0 |  |  |  |
Other Postretirement Benefits |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |  |  |  | 216.3 |  | 262.5 |  | 231.4
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) |  |  |  |  |  | 5.0 |  | (18.9) |  |
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) |  | (4.0) | (4.4) |  |  | (4.0) |  | (4.4) |  | (4.9)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) |  |  |  |  |  | 7.5 |  | 5.3 |  | $ 8.0
Employer contributions |  |  |  |  |  | 21.0 |  | 31.6 |  |
Expected contribution |  |  |  |  |  | 20.6 |  |  |  |
Funded status of plan |  |  |  |  |  | 276.2 |  | 293.8 |  |
Liability, Defined Benefit Plan | [1] |  |  |  |  | $ 276.2 |  | $ 293.8 |  |
Expected return on plan assets |  |  |  |  |  | 5.80% |  | 6.99% |  | 7.85%
Settlement loss |  | $ 0.0 | $ 0.0 |  |  | $ 0.0 |  | $ 0.0 |  | $ 0.0
Significant Unobservable Inputs (Level 3) [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |  |  |  | $ 86.1 |  | 98.9 |  | $ 73.1
Percentage of investments |  |  |  |  |  | 4.00% |  |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Other Postretirement Benefits |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |  |  |  |  |  | 0.0 |  |
Scenario, Forecast | Pension Plan |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) |  |  |  |  | $ 45.5 |  |  |  |  |
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) |  |  |  |  | (0.2) |  |  |  |  |
Amortization of transition obligation |  |  |  |  | 0.0 |  |  |  |  |
Scenario, Forecast | Other Postretirement Benefits |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) |  |  |  |  | 2.4 |  |  |  |  |
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) |  |  |  |  | 3.2 |  |  |  |  |
Amortization of transition obligation |  |  |  |  | $ 0.0 |  |  |  |  |
Unrecognized Pension Benefit And Other Postretirement Benefit Costs |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  |  |  | $ 798.3 |  | 733.5 |  |
Regulatory Liabilities |  |  |  |  |  | 0.1 |  | 0.1 |  |
International Equities | Pension Plan |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |  |  |  | 165.5 |  | 351.0 |  |
International Equities | Other Postretirement Benefits |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |  |  |  | 17.5 |  | 39.8 |  |
Commingled Funds | Short-Term Money Markets | Pension Plan |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |  |  |  | 18.3 | [2] | 34.1 | [3] |
Commingled Funds | Short-Term Money Markets | Other Postretirement Benefits |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |  |  |  | 5.2 | [2] | 9.4 | [3] |
Commingled Funds | United States Equities | Pension Plan |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |  |  |  | 245.2 | [2] | 466.6 | [3] |
Commingled Funds | United States Equities | Other Postretirement Benefits |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |  |  |  | 10.4 | [2] | 12.2 | [3] |
Commingled Funds | International Equities | Pension Plan |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |  |  |  | $ 122.3 | [2] | $ 265.1 | [3] |
[1] | recognize our Consolidated Balance Sheets underfunded and overfunded status of our various defined benefit postretirement plans, measured as the difference between the fair value of the plan assets and the benefit obligation.
[2] | This class of investments is measured at fair value using the net asset value per share and has not been classified in the fair value hierarchy.
[3] | This class of investments is measured at fair value using the net asset value per share and has not been classified in the fair value hierarchy.

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