Postretirement Health Care and Life Insurance Benefits Pre-Tax Amounts Recorded in Accumulated Other Comprehensive Loss (Details) - Postretirement Health Care and Life Insurance Benefits $ in Millions	3 Months Ended	12 Months Ended
	Apr. 01, 2017 USD ($)	Dec. 31, 2020 USD ($) years	Dec. 31, 2019 USD ($) years	Dec. 31, 2018 USD ($)	Dec. 31, 2017 years
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax	$ 0.0		$ 0.0	$ (51.7)
Amortization:
Actuarial loss	(5.5)		0.0	0.0
Prior service credit	2.3		8.7	0.0
Settlement related to the Patriot bankruptcy:
Total recorded in “Accumulated other comprehensive income”	$ (3.2)		$ 8.7	$ (51.7)
Percentage of actuarial gains and losses amortized	0.00%
Remaining service period for remaining eligible employees | years			5.9		10.3
Scenario, Forecast
Settlement related to the Patriot bankruptcy:
Remaining service period for remaining eligible employees | years		4.9
Defined Benefit Plan, Expected Amortization of Prior Service Cost (Credit), Next Fiscal Year		$ 8.7