Employee Retirement Plans - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Retirement Benefits [Abstract]
Actual employer contributions to defined benefit plans	$ 3.0	$ 0.9	$ 3.7	$ 0.9
Expected full year contributions by the employer to defined benefit plans	18.4		18.4
Contributions to multiemployer plan	$ 0.5	$ 0.3	1.1	$ 0.9
Expected full year contributions by the employer to the multiemployer plan			$ 2.3