Property and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Properties [Line items]
Cost	$ 1,346.0	$ 1,132.5
Less accumulated depreciation and amortization	996.2	896.8
Properties, net	349.8	235.7
Assets under capital lease, net		2.5
Assets under capital lease, accumulated depreciation		2.2
Depreciation
Depreciation expense	99.8	78.2	$ 77.6
Aggregate Principle of IRB	103.9	87.6
Land
Properties [Line items]
Cost	52.9	34.2
Buildings
Properties [Line items]
Cost	198.0	160.3
Technology equipment
Properties [Line items]
Cost	248.4	213.0
Software
Properties [Line items]
Cost	591.4	508.2
Less accumulated depreciation and amortization	500.2	453.5
Software development and maintenance costs
Construction in Progress, Gross	11.5	14.9
Software | Business combinations
Properties [Line items]
Cost	83.6	56.5
Less accumulated depreciation and amortization	52.8	44.5
Furniture, fixtures and other equipment
Properties [Line items]
Cost	151.6	122.1
Leasehold improvements
Properties [Line items]
Cost	91.6	76.4
Construction-in-progress
Properties [Line items]
Cost	12.1	18.3
Capitalized software costs
Properties [Line items]
Cost	324.0	296.4
Less accumulated depreciation and amortization	287.9	266.0
Software development and maintenance costs
Capitalized software development costs	24.1	21.5	$ 23.3
Purchased Software
Properties [Line items]
Cost	183.8	155.3
Less accumulated depreciation and amortization	$ 159.5	$ 143.0