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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%

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