Other Postretirement Benefit Plans - AOCL (Details) - Accrued postretirement liability - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Amounts recognized in accumulated other comprehensive income:
Net prior service credit	$ (91.5)	$ (98.2)
Accumulated other comprehensive income	$ (91.5)	$ (98.2)	$ (10.8)

Amount, before tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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-Publisher FASB
-Name Accounting Standards Codification
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-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant