Supplementary Data Property and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Furniture and equipment	$ 634.8	$ 604.2
Leasehold improvements	641.5	599.8
Internal-use computer software	331.3	306.5
Land and buildings	79.0	73.1
Total property and equipment, gross	1,686.6	1,583.6
Less: accumulated depreciation	(1,036.2)	(961.6)
Total property and equipment, net	650.4	622.0
Depreciation and amortization expense for property and equipment	$ 135.9	$ 138.3	$ 130.9