Retirement Benefits - Schedule of Estimated Future Benefit Payments (Details) $ in Millions	Dec. 31, 2020 USD ($)
Pension Benefits
Expected Benefit Payments
2021	$ 106.9
2022	103.8
2023	100.6
2024	97.1
2025	93.4
Years 2026 - 2030	409.1
Medical and Life Insurance Benefits
Expected Benefit Payments
2021	3.4
2022	3.0
2023	2.7
2024	2.4
2025	2.1
Years 2026 - 2030	7.4
Gross Benefit Payments
2021	3.5
2022	3.1
2023	2.8
2024	2.5
2025	2.2
Years 2026 - 2030	7.6
Medicare D Subsidy
2021	0.1
2022	0.1
2023	0.1
2024	0.1
2025	0.1
Years 2026 - 2030	$ 0.2

Defined Benefit Plan, Expected Future Gross Benefit Payment

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Gross Projected Postretirement Benefits Payments Before Medicare Part D Subsidies Five Fiscal Years Thereafter

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Gross Projected Postretirement Benefits Payments Before Medicare Part D Subsidies Next Twelve Months

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Gross Projected Postretirement Benefits Payments Before Medicare Part D Subsidies Year Five

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Gross Projected Postretirement Benefits Payments Before Medicare Part D Subsidies Year Four

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Gross Projected Postretirement Benefits Payments Before Medicare Part D Subsidies Year Three

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Gross Projected Postretirement Benefits Payments Before Medicare Part D Subsidies Year Two

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Amount of benefit for defined benefit plan expected to be paid in five fiscal years after fifth fiscal year following current fiscal year.

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Amount of benefit for defined benefit plan expected to be paid in next fiscal year following current fiscal year.

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Amount of benefit for defined benefit plan expected to be paid in fifth fiscal year following current fiscal year.

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Amount of benefit for defined benefit plan expected to be paid in fourth fiscal year following current fiscal year.

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Amount of benefit for defined benefit plan expected to be paid in third fiscal year following current fiscal year.

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Amount of benefit for defined benefit plan expected to be paid in second fiscal year following current fiscal year.

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Amount of prescription drug subsidy receipt expected in third fiscal year following current fiscal year.

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