Contingencies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loss Contingency [Abstract]
Rent expense	$ 58.6	$ 44.9	$ 40.2
Letters of credit outstanding	75.9
Outstanding surety bonds	$ 573.4