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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%

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