STOCKHOLDERS' EQUITY: Future expense, by award type (Details) $ in Millions	Sep. 30, 2019 USD ($)
Share-based compensation
Future expense for performance stock option units	$ 203.6
2020
Share-based compensation
Future expense for performance stock option units	42.0
2021
Share-based compensation
Future expense for performance stock option units	73.0
2022
Share-based compensation
Future expense for performance stock option units	61.0
2023
Share-based compensation
Future expense for performance stock option units	24.6
2024
Share-based compensation
Future expense for performance stock option units	3.0
Stock options
Share-based compensation
Share-based compensation not yet recognized	5.8
Stock options | 2020
Share-based compensation
Share-based compensation not yet recognized	1.9
Stock options | 2021
Share-based compensation
Share-based compensation not yet recognized	2.4
Stock options | 2022
Share-based compensation
Share-based compensation not yet recognized	1.2
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
Restricted stock units
Share-based compensation
Future share-based compensation expense expected	137.3
Restricted stock units | 2020
Share-based compensation
Future share-based compensation expense expected	28.4
Restricted stock units | 2021
Share-based compensation
Future share-based compensation expense expected	47.2
Restricted stock units | 2022
Share-based compensation
Future share-based compensation expense expected	36.5
Restricted stock units | 2023
Share-based compensation
Future share-based compensation expense expected	22.2
Restricted stock units | 2024
Share-based compensation
Future share-based compensation expense expected	3.0
Consideration Holdback | Data Plus Math Corporation
Share-based compensation
Future share-based compensation expense expected	22.7
Consideration Holdback | Data Plus Math Corporation | 2020
Share-based compensation
Future share-based compensation expense expected	4.1
Consideration Holdback | Data Plus Math Corporation | 2021
Share-based compensation
Future share-based compensation expense expected	8.3
Consideration Holdback | Data Plus Math Corporation | 2022
Share-based compensation
Future share-based compensation expense expected	8.2
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
Pacific Data Partners, LLC Agreement Plan | Pacific Data Partners LLC
Share-based compensation
Future share-based compensation expense expected	37.8
Pacific Data Partners, LLC Agreement Plan | Pacific Data Partners LLC | 2020
Share-based compensation
Future share-based compensation expense expected	7.6
Pacific Data Partners, LLC Agreement Plan | Pacific Data Partners LLC | 2021
Share-based compensation
Future share-based compensation expense expected	15.1
Pacific Data Partners, LLC Agreement Plan | Pacific Data Partners LLC | 2022
Share-based compensation
Future share-based compensation expense expected	15.1
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