Pension and Postretirement Benefit Costs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Components of net periodic benefit cost
Net periodic cost (benefit)	$ 4.9	$ 4.5
Defined benefit pension plans
Components of net periodic benefit cost
Service cost for benefits earned	0.5	0.6
Interest cost on projected benefit obligation	8.3	7.8
Expected return on plan assets	(7.8)	(10.7)
Net periodic cost (benefit)	$ 1.0	(2.3)
Other Defined Benefit Plan Information [Abstract]
Funding threshold	80.00%
Postretirement benefit plans
Components of net periodic benefit cost
Service cost for benefits earned	$ 1.2	2.0
Interest cost on projected benefit obligation	6.3	7.1
Expected return on plan assets	(0.1)	0.0
Amortization of prior service cost and net actuarial loss	(2.2)	0.0
Net periodic cost (benefit)	$ 5.2	$ 9.1