Employee Retirement Plans - Components of net retirement cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Components of Net Retirement Cost
Service cost	$ 0.0	$ 0.0
Interest	4.9	4.5
Expected return on plan assets	(5.7)	(6.8)
Amortization of actuarial loss	1.1	1.2
Defined benefit expense	0.3	(1.1)
Profit sharing	2.2	2.5
Net expense	$ 2.5	$ 1.4