Employee Retirement Plans - Components of net retirement cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Components of Net Retirement Cost
Service cost	$ 0.0	$ 0.0
Interest	3.7	4.9
Expected return on plan assets	(5.2)	(5.7)
Amortization of actuarial loss	1.5	1.1
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	0.3	0.0
Net periodic benefit cost	0.3	0.3
Profit sharing	2.7	2.2
Net expense	$ 3.0	$ 2.5