Benefit Plans - Amounts Recognized in Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Pension benefits
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net actuarial loss (gain)	$ 67.3	$ 69.4
Prior service cost (credit)	0.8	0.8
Total	68.1	70.2
Other retirement benefits
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net actuarial loss (gain)	(5.4)	(7.0)
Prior service cost (credit)	(0.4)	(1.0)
Total	$ (5.8)	$ (8.0)

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
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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
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-Paragraph 1
-Subparagraph (j)
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition

Amount, before tax, of accumulated other comprehensive (income) loss for 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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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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