SHARE-BASED COMPENSATION: Non vested time vesting restricted stock unit activity (Details) - Restricted stock units - USD ($) $ / shares in Units, $ in Millions	6 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2018
Restricted Stock Unit Activity - Other disclosures
Share-based compensation expense	$ 17.5	$ 14.9
Time Based Vesting
Non-vested restricted stock unit activity
Outstanding at the beginning of the period (in shares)	2,702,497
Granted (in shares)	1,383,409
Vested (in shares)	(745,573)
Forfeited or cancelled (in shares)	(200,222)
Outstanding at the end of the period (in shares)	3,140,111		2,702,497
Non-vested restricted stock units, Weighted average fair value per share at grant date
Outstanding at the beginning of the period (in dollars per share)	$ 24.60
Granted (in dollars per share)	29.15
Vested (in dollars per share)	23.94
Forfeited or cancelled (in dollars per share)	25.18
Outstanding at the end of the period (in dollars per share)	$ 26.73		$ 24.60
Weighted-average remaining contractual term
Weighted average remaining contractual term (in years)	2 years 8 months 15 days		2 years 4 months 2 days
Restricted Stock Unit Activity - Other disclosures
Aggregate fair value of restricted stock units granted	$ 40.3
Performance stock
Non-vested restricted stock unit activity
Outstanding at the beginning of the period (in shares)	561,018
Granted (in shares)	186,539
Forfeited or cancelled (in shares)	(110,536)
Outstanding at the end of the period (in shares)	637,021		561,018
Non-vested restricted stock units, Weighted average fair value per share at grant date
Outstanding at the beginning of the period (in dollars per share)	$ 25.58
Granted (in dollars per share)	31.07
Forfeited or cancelled (in dollars per share)	25.08
Outstanding at the end of the period (in dollars per share)	$ 27.28		$ 25.58
Weighted-average remaining contractual term
Weighted average remaining contractual term (in years)	1 year 6 months 21 days		1 year 8 months 1 day
Restricted Stock Unit Activity - Other disclosures
Aggregate fair value of restricted stock units granted	$ 5.8
Performance Shares Vesting Total Shareholder Return
Non-vested restricted stock unit activity
Granted (in shares)	186,539
Share-based Compensation Award, Tranche One
Restricted Stock Unit Activity - Other disclosures
Future share-based compensation expense expected	$ 20.5
Share-based Compensation Award, Tranche Two
Restricted Stock Unit Activity - Other disclosures
Future share-based compensation expense expected	29.2
Share-based Compensation Award, Tranche Three
Restricted Stock Unit Activity - Other disclosures
Future share-based compensation expense expected	19.8
Share-based Compensation Award, Tranche Four
Restricted Stock Unit Activity - Other disclosures
Future share-based compensation expense expected	10.5
Share-based Compensation Award, Tranche Five
Restricted Stock Unit Activity - Other disclosures
Future share-based compensation expense expected	$ 1.4
Vesting in four years
Share-based compensation
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	4 years
Non-vested restricted stock unit activity
Granted (in shares)	197,115
Partial cliff vest tranche one
Non-vested restricted stock unit activity
Granted (in shares)	1,172,263
Partial cliff vesting tranche two
Share-based compensation
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	3 years
Vesting in one year
Share-based compensation
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	1 year
Non-vested restricted stock unit activity
Granted (in shares)	14,031
Minimum | Performance Shares Vesting Total Shareholder Return
Restricted Stock Unit Activity - Other disclosures
Vesting percentage	25.00%
Maximum | Performance Shares Vesting Total Shareholder Return
Restricted Stock Unit Activity - Other disclosures
Vesting percentage	200.00%