STOCK-BASED COMPENSATION: Future expense, by award type (Details) $ in Millions	Dec. 31, 2020 USD ($)
Share-based compensation
Total	$ 207.1
2021
Share-based compensation
Total	67.7
2022
Share-based compensation
Total	63.6
2023
Share-based compensation
Total	47.1
2024
Share-based compensation
Total	24.3
2025
Share-based compensation
Total	4.4
Stock options
Share-based compensation
Share-based compensation not yet recognized	1.7
Stock options | 2021
Share-based compensation
Share-based compensation not yet recognized	0.3
Stock options | 2022
Share-based compensation
Share-based compensation not yet recognized	1.1
Stock options | 2023
Share-based compensation
Share-based compensation not yet recognized	0.3
Stock options | 2024
Share-based compensation
Share-based compensation not yet recognized	0.0
Stock options | 2025
Share-based compensation
Share-based compensation not yet recognized	0.0
Restricted stock units
Share-based compensation
Future share-based compensation expense expected	174.7
Restricted stock units | 2021
Share-based compensation
Future share-based compensation expense expected	59.4
Restricted stock units | 2022
Share-based compensation
Future share-based compensation expense expected	42.9
Restricted stock units | 2023
Share-based compensation
Future share-based compensation expense expected	43.9
Restricted stock units | 2024
Share-based compensation
Future share-based compensation expense expected	24.1
Restricted stock units | 2025
Share-based compensation
Future share-based compensation expense expected	4.4
Consideration Holdback | Data Plus Math Corporation
Share-based compensation
Future share-based compensation expense expected	12.2
Consideration Holdback | Data Plus Math Corporation | 2021
Share-based compensation
Future share-based compensation expense expected	2.0
Consideration Holdback | Data Plus Math Corporation | 2022
Share-based compensation
Future share-based compensation expense expected	8.1
Consideration Holdback | Data Plus Math Corporation | 2023
Share-based compensation
Future share-based compensation expense expected	2.1
Consideration Holdback | Data Plus Math Corporation | 2024
Share-based compensation
Future share-based compensation expense expected	0.0
Consideration Holdback | Data Plus Math Corporation | 2025
Share-based compensation
Future share-based compensation expense expected	0.0
Pacific Data Partners, LLC Agreement Plan | Pacific Data Partners LLC
Share-based compensation
Future share-based compensation expense expected	13.8
Pacific Data Partners, LLC Agreement Plan | Pacific Data Partners LLC | 2021
Share-based compensation
Future share-based compensation expense expected	4.6
Pacific Data Partners, LLC Agreement Plan | Pacific Data Partners LLC | 2022
Share-based compensation
Future share-based compensation expense expected	9.2
Pacific Data Partners, LLC Agreement Plan | Pacific Data Partners LLC | 2023
Share-based compensation
Future share-based compensation expense expected	0.0
Pacific Data Partners, LLC Agreement Plan | Pacific Data Partners LLC | 2024
Share-based compensation
Future share-based compensation expense expected	0.0
Pacific Data Partners, LLC Agreement Plan | Pacific Data Partners LLC | 2025
Share-based compensation
Future share-based compensation expense expected	0.0
Acuity Performance Earnout | Acuity Data
Share-based compensation
Future share-based compensation expense expected	3.7
Acuity Performance Earnout | Acuity Data | 2021
Share-based compensation
Future share-based compensation expense expected	0.8
Acuity Performance Earnout | Acuity Data | 2022
Share-based compensation
Future share-based compensation expense expected	1.9
Acuity Performance Earnout | Acuity Data | 2023
Share-based compensation
Future share-based compensation expense expected	0.8
Acuity Performance Earnout | Acuity Data | 2024
Share-based compensation
Future share-based compensation expense expected	0.2
Acuity Performance Earnout | Acuity Data | 2025
Share-based compensation
Future share-based compensation expense expected	0.0
Other stock-based compensation
Share-based compensation
Future share-based compensation expense expected	1.0
Other stock-based compensation | 2021
Share-based compensation
Future share-based compensation expense expected	0.6
Other stock-based compensation | 2022
Share-based compensation
Future share-based compensation expense expected	0.4
Other stock-based compensation | 2023
Share-based compensation
Future share-based compensation expense expected	0.0
Other stock-based compensation | 2024
Share-based compensation
Future share-based compensation expense expected	0.0
Other stock-based compensation | 2025
Share-based compensation
Future share-based compensation expense expected	$ 0.0