Postretirement Health Care and Life Insurance Benefits - Net Periodic Postretirement Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Components of net periodic postretirement benefit cost
Net periodic postretirement benefit (benefit) cost	$ (38.3)	$ (1.8)	$ 19.4
Postretirement Health Care and Life Insurance Benefits
Components of net periodic postretirement benefit cost
Service cost for benefits earned	1.0	3.8	4.8
Interest cost on accumulated postretirement benefit obligation	10.5	20.2	25.1
Expected return on plan assets	(1.0)	(1.5)	(0.5)
Amortization of prior service credit	(46.4)	(17.3)	(8.7)
Net actuarial (gain) loss	(54.5)	16.5	78.3
Net periodic postretirement benefit (benefit) cost	$ (90.4)	$ 21.7	$ 99.0