Stock-Based Compensation - Stock-based Compensation Expense by Award Type and Expense Category (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Feb. 03, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense (credit)	$ 26.2	$ 15.7	$ 62.3	$ 40.8
Cost of services and product development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense (credit)	8.0	7.1	21.7	19.2
Selling, general and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense (credit)	18.2	8.6	40.6	21.6
Stock appreciation rights
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense (credit)	2.0	1.9	4.0	3.6
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense (credit)	24.0	13.6	57.9	36.8
Common stock equivalents
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense (credit)	0.2	0.2	0.4	0.4
Retirement eligible employees equity award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense (credit)	$ 11.3	$ 5.2	32.8	$ 16.8
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%