Employee Retirement Plans - Components of net retirement cost - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Expense Components
Service cost	$ 0.2	$ 0.4	$ 0.5
Interest	19.6	20.8	20.0
Expected return on plan assets	(27.2)	(27.2)	(30.5)
Amortization of actuarial loss	4.9	5.1	5.0
Defined benefit expense	(2.5)	(0.9)	(5.0)
Profit sharing	15.5	15.2	18.7
Multiemployer plan	2.1	2.3	2.4
Net expense	$ 15.1	$ 16.6	$ 16.1