Employee Retirement Plans - Components of net retirement cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Components of Net Retirement Cost
Service cost	$ 0.1	$ 0.1	$ 0.1	$ 0.2
Interest	4.6	4.9	9.1	9.8
Expected return on plan assets	(6.9)	(6.8)	(13.7)	(13.6)
Amortization of actuarial loss	1.1	1.2	2.3	2.4
Defined benefit expense	(1.1)	(0.6)	(2.2)	(1.2)
Profit sharing	4.4	3.8	8.8	7.8
Multiemployer plan	0.5	0.4	1.1	1.0
Net expense	$ 3.8	$ 3.6	$ 7.7	$ 7.6