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CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME Parenthetical - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Pension Plan and Restoration of Retirement Income Plan: |  |  |
Amortization of deferred net loss, net of tax of $1.2, $1.1 and $1.1, respectively | $ 1.2 | $ 1.1 | $ 1.1
Net loss arising during the period, net of tax of ($1.7), ($2.6) and ($4.7), respectively | (1.7) | (2.6) | (4.7)
Postretirement plans: |  |  |
Amortization of deferred net gain, net of tax of $0.0, $0.0 and $0.0, respectively | 0.0 | 0.0 | 0.0
Net gain (loss) arising during the period, net of tax of ($0.8), ($0.1) and $0.7, respectively | (0.8) | (0.1) | 0.7
Amortization of prior service credit, net of tax of ($0.6), ($0.6) and ($0.6), respectively | (0.6) | (0.6) | (0.6)
Pension Plans [Member] |  |  |
Pension Plan and Restoration of Retirement Income Plan: |  |  |
Settlement cost, net of tax of $0.7, $2.7 and $1.6, respectively | 0.7 | 2.7 | 1.6
Other Postretirement Benefits Plan [Member] |  |  |
Pension Plan and Restoration of Retirement Income Plan: |  |  |
Settlement cost, net of tax of $0.7, $2.7 and $1.6, respectively | $ (0.1) | $ 0.0 |
Postretirement plans: |  |  |
Curtailment cost, net of tax of ($0.1), $0.0 and $0.0, respectively |  |  | $ 0.0

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