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	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Prior service credit arising during year		$ (185.4)	$ 0.0	$ (51.7)
Amortization:
Prior service credit		17.3	8.7	0.0
Total recorded in “Accumulated other comprehensive income”		$ (168.1)	$ 8.7	$ (51.7)
Remaining service period for remaining eligible employees		4 years 10 months 24 days
Defined benefit plan, expected amortization of prior service cost (credit), next fiscal year		$ 43.9
Scenario, Forecast
Amortization:
Remaining service period for remaining eligible employees	3 years 10 months 24 days