Stock-Based Compensation - Stock-based Compensation Expense by Award Type and Expense Category (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Feb. 03, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense (credit)	$ 19.4	$ 15.5	$ 81.7	$ 56.3
Cost of services and product development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense (credit)	6.9	6.1	28.6	25.3
Selling, general and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense (credit)	12.5	9.4	53.1	31.0
Stock appreciation rights
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense (credit)	2.1	2.1	6.1	5.7
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense (credit)	17.1	13.2	75.0	50.0
Common stock equivalents
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense (credit)	0.2	0.2	0.6	0.6
Retirement eligible employees equity award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense (credit)	$ 5.5	$ 3.4	38.3	$ 20.2
Performance-based RSUs | Executive officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense (credit)			$ 6.5
Target performance percentage	50.00%		50.00%		95.00%