Pension and Postretirement Benefit Plans - Share of Net Periodic Cost Components (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Amortization of:
Prior service benefit	$ (5.5)
Pension
Components of net periodic cost:
Service cost		$ 5.1	$ 5.7	$ 6.2
Interest cost		10.7	11.4	11.9
Expected return on plan assets		(18.0)	(16.7)	(15.1)
Amortization of:
Prior service benefit		0.0	0.0	0.0
Net actuarial loss		8.2	7.8	9.2
Net periodic cost (benefit)		6.0	8.2	12.2
Postretirement
Components of net periodic cost:
Service cost		0.0	0.0	0.0
Interest cost		0.6	0.8	0.9
Expected return on plan assets		0.0	0.0	0.0
Amortization of:
Prior service benefit		(5.5)	(5.5)	(5.5)
Net actuarial loss		0.3	0.2	0.2
Net periodic cost (benefit)		$ (4.6)	$ (4.5)	$ (4.4)