Postretirement Health Care and Life Insurance Benefits - Net Periodic Postretirement Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of net periodic postretirement benefit cost
Net periodic benefit (credit) costs, excluding service cost	$ (1.8)	$ 19.4	$ 18.1
Postretirement Health Care and Life Insurance Benefits
Components of net periodic postretirement benefit cost
Service cost for benefits earned	3.8	4.8	8.2
Interest cost on accumulated postretirement benefit obligation	20.2	25.1	28.3
Expected return on plan assets	(1.5)	(0.5)	0.0
Amortization of prior service credit	(17.3)	(8.7)	0.0
Amortization of actuarial loss	16.5	78.3	(128.4)
Net periodic benefit (credit) costs, excluding service cost	$ 21.7	$ 99.0	$ (91.9)