STOCK-BASED COMPENSATION: Future expense, by award type (Details) $ in Millions	Sep. 30, 2020 USD ($)
Share-based compensation
Total	$ 198.2
2021
Share-based compensation
Total	48.1
2022
Share-based compensation
Total	76.9
2023
Share-based compensation
Total	47.0
2024
Share-based compensation
Total	23.1
2025
Share-based compensation
Total	3.1
Stock options
Share-based compensation
Share-based compensation not yet recognized	2.2
Stock options | 2021
Share-based compensation
Share-based compensation not yet recognized	0.8
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	158.9
Restricted stock units | 2021
Share-based compensation
Future share-based compensation expense expected	32.5
Restricted stock units | 2022
Share-based compensation
Future share-based compensation expense expected	56.6
Restricted stock units | 2023
Share-based compensation
Future share-based compensation expense expected	43.8
Restricted stock units | 2024
Share-based compensation
Future share-based compensation expense expected	22.9
Restricted stock units | 2025
Share-based compensation
Future share-based compensation expense expected	3.1
Consideration Holdback | Data Plus Math Corporation
Share-based compensation
Future share-based compensation expense expected	14.2
Consideration Holdback | Data Plus Math Corporation | 2021
Share-based compensation
Future share-based compensation expense expected	4.1
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	2.0
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	18.4
Pacific Data Partners, LLC Agreement Plan | Pacific Data Partners LLC | 2021
Share-based compensation
Future share-based compensation expense expected	9.2
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	4.5
Acuity Performance Earnout | Acuity Data | 2021
Share-based compensation
Future share-based compensation expense expected	1.5
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	0.9
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