Employee Retirement Plans - Components of net retirement cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Components of Net Retirement Cost
Service cost	$ 0.1	$ 0.1	$ 0.1	$ 0.1
Interest	4.9	4.6	9.8	9.1
Expected return on plan assets	(5.8)	(6.9)	(11.5)	(13.7)
Amortization of actuarial loss	1.1	1.1	2.2	2.3
Net periodic benefit cost	0.3	(1.1)	0.6	(2.2)
Profit sharing	3.0	2.5	5.2	5.0
Net expense	$ 3.3	$ 1.4	$ 5.8	$ 2.8