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Defined Benefit Pension and Other Postretirement Plans (Tables) | 6 Months Ended
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Mar. 31, 2018
Retirement Benefits [Abstract] |
Components of Net Periodic Pension Expense and Other Postretirement Benefit Costs | Net periodic pension expense and other postretirement benefit costs include the following components:
|  | Pension Benefits |  | Other Postretirement Benefits
Three Months Ended March 31, |  | 2018 |  | 2017 |  | 2018 |  | 2017
Service cost |  | $ | 2.8 |  |  | $ | 3.0 |  |  | $ | 0.2 |  |  | $ | 0.3 |
Interest cost |  | 6.5 |  |  | 6.1 |  |  | 0.2 |  |  | 0.2 |
Expected return on assets |  | (8.6 | ) |  | (8.3 | ) |  | (0.2 | ) |  | (0.1 | )
Amortization of: |  |  |  |  |  |  |  |
Prior service cost (benefit) |  | 0.1 |  |  | — |  |  | — |  |  | (0.2 | )
Actuarial loss |  | 3.3 |  |  | 4.2 |  |  | — |  |  | — |
Net benefit cost |  | 4.1 |  |  | 5.0 |  |  | 0.2 |  |  | 0.2 |
Change in associated regulatory liabilities |  | — |  |  | — |  |  | (0.1 | ) |  | (0.1 | )
Net benefit cost after change in regulatory liabilities |  | $ | 4.1 |  |  | $ | 5.0 |  |  | $ | 0.1 |  |  | $ | 0.1 |
|  | Pension Benefits |  | Other Postretirement Benefits
Six Months Ended March 31, |  | 2018 |  | 2017 |  | 2018 |  | 2017
Service cost |  | $ | 5.6 |  |  | $ | 6.0 |  |  | $ | 0.4 |  |  | $ | 0.5 |
Interest cost |  | 13.0 |  |  | 12.3 |  |  | 0.4 |  |  | 0.4 |
Expected return on assets |  | (17.2 | ) |  | (16.6 | ) |  | (0.4 | ) |  | (0.3 | )
Amortization of: |  |  |  |  |  |  |  |
Prior service cost (benefit) |  | 0.2 |  |  | 0.1 |  |  | (0.1 | ) |  | (0.3 | )
Actuarial loss |  | 6.6 |  |  | 8.3 |  |  | — |  |  | 0.1 |
Net benefit cost |  | 8.2 |  |  | 10.1 |  |  | 0.3 |  |  | 0.4 |
Change in associated regulatory liabilities |  | — |  |  | — |  |  | (0.2 | ) |  | (0.2 | )
Net benefit cost after change in regulatory liabilities |  | $ | 8.2 |  |  | $ | 10.1 |  |  | $ | 0.1 |  |  | $ | 0.2 |

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- Definition Tabular disclosure of the components of net benefit costs for pension plans and/or other employee benefit plans including service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) recognized due to settlements or curtailments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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