Postretirement Health Care and Life Insurance Benefits Net Periodic Postretirement Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Components of net periodic postretirement benefit cost
Net periodic postretirement benefit cost	$ 14.4	$ 21.9	$ 19.4	$ 18.1
Postretirement Health Care and Life Insurance Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated postretirement benefit obligation at end of period		783.3	659.9	595.4
Components of net periodic postretirement benefit cost
Service cost for benefits earned	2.3	6.9	4.8	8.2
Interest cost on accumulated postretirement benefit obligation	8.4	24.2	25.1	28.3
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	0.0	0.0	(0.5)	0.0
Amortization of prior service credit	(2.3)	0.0	(8.7)	0.0
Amortization of actuarial loss	5.5	0.0	0.0	0.0
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss), Net	0.0	(22.0)	78.3	(128.4)
Net periodic postretirement benefit cost	$ 13.9	$ 9.1	$ 99.0	$ (91.9)