Pension, Health Care and Postretirement Benefits Other Than Pensions - Summary of the Components of Net Periodic Benefit Cost and Cumulative Other Comprehensive Loss Related to Postretirement Benefits Other than Pensions (Details) - Postretirement Benefits Other than Pensions - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net pension cost (credit):
Service cost	$ 1.5	$ 2.0	$ 2.1
Interest cost	11.2	10.2	10.8
Amortization of actuarial losses	0.5	2.3
Amortization of prior service credit	(5.0)	(6.6)	(6.6)
Net periodic benefit cost	8.2	7.9	6.3
Settlement credit			(9.3)
Net pension cost	8.2	7.9	(3.0)
Other changes in projected benefit obligation recognized in AOCI (before taxes):
Net actuarial loss (gain) arising during the year	12.8	(9.0)	11.6
Prior service cost arising during the year		(0.1)
Amortization of actuarial losses	(0.5)	(2.3)
Settlement cost			9.3
Amortization of prior service credit	5.0	6.6	6.6
Total recognized in AOCI	17.3	(4.8)	27.5
Total recognized in net pension cost and AOCI	$ 25.5	$ 3.1	$ 24.5

Pension, Health Care and Postretirement Benefits Other Than Pensions - Summary of the Components of Net Periodic Benefit Cost and Cumulative Other Comprehensive Loss Related to Postretirement Benefits Other than Pensions (Details) - Postretirement Benefits Other than Pensions - USD ($)
$ in Millions

12 Months Ended

Dec. 31, 2019

Dec. 31, 2018

Dec. 31, 2017

Net pension cost (credit):

Service cost

$ 1.5

$ 2.0

$ 2.1

Interest cost

11.2

10.2