Employee Retirement Plans - Components of net retirement cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Expense Components
Service cost	$ 0.1	$ 0.2	$ 0.4
Interest	18.3	19.6	20.8
Expected return on plan assets	(27.4)	(27.2)	(27.2)
Amortization of actuarial loss	4.8	4.9	5.1
Defined Benefit Plan, Other Cost (Credit)	0.6	0.0	0.0
Net periodic benefit cost	(3.6)	(2.5)	(0.9)
Profit sharing	11.1	7.7	7.2
Net expense	$ 7.5	$ 5.2	$ 6.3