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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
	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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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.

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Amount, before tax, of cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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