Employee Retirement Plans - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Retirement Benefits [Abstract]
Actual employer contributions to defined benefit plans	$ 28.0	$ 0.8	$ 31.7	$ 1.7
Expected full year contributions by the employer to defined benefit plans	31.7		31.7
Contributions to multiemployer plan	$ 0.5	$ 0.5	1.6	$ 1.4
Expected full year contributions by the employer to the multiemployer plan			$ 2.3