Retirement Benefits - Narrative (Details) $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2019 employee
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer	$ 3.3
Participants affected due to plan freeze (fewer than) | employee		60
Recognized loss	9.7
Previously deferred pension cost	10.7
Increase in plan assets	1.0
Funded status of plan	10.9
Equities
Defined Benefit Plan Disclosure [Line Items]
Trading securities	1.1
Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Trading securities	9.8
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected contribution for next fiscal year	3.4
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected contribution for next fiscal year	$ 1.0