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Postretirement Health Care and Life Insurance Benefits - Pre-Tax Amounts Recorded in Accumulated Other Comprehensive Loss (Details) - Postretirement Health Care and Life Insurance Benefits - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items] |  |  |
Prior service credit arising during year | $ (139.5) | $ (185.4) | $ 0.0
Amortization: |  |  |
Prior service credit | 46.4 | 17.3 | 8.7
Total recorded in “Accumulated other comprehensive income” | (93.1) | $ (168.1) | $ 8.7
Defined benefit plan, expected amortization of prior service cost (credit), next fiscal year | $ 53.8 |  |

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- Definition Amount included in accumulated other comprehensive (income) loss for prior service cost (credit) expected to be recognized in net periodic benefit cost (credit) for fiscal year following most recent annual statement of financial position. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (s) -URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920 + Details
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