Accrued Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Accrued Liabilities [Abstract]
Salaries, benefits and related expenses	$ 441.7	$ 499.0
Office and related expenses	53.2	46.7
Acquisition obligations	42.0	77.5
Interest	16.4	17.3
Other	121.4	153.5
Total accrued liabilities	$ 674.7	$ 794.0