Employee Retirement Plans - Components of net retirement cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Components of Net Retirement Cost
Service cost	$ 0.0	$ 0.0	$ 0.1	$ 0.2
Interest	4.6	4.9	13.7	14.7
Expected return on plan assets	(6.9)	(6.8)	(20.6)	(20.4)
Amortization of actuarial loss	1.3	1.4	3.6	3.8
Defined benefit expense	(1.0)	(0.5)	(3.2)	(1.7)
Profit sharing	4.7	3.9	13.5	11.7
Multiemployer plan	0.5	0.6	1.6	1.6
Net expense	$ 4.2	$ 4.0	$ 11.9	$ 11.6