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SHARE-BASED COMPENSATION: Non vested time vesting restricted stock unit activity (Details)
$ / shares in Units, $ in Millions
6 Months Ended 12 Months Ended
Sep. 30, 2017
USD ($)
tranche
$ / shares
shares
Sep. 30, 2016
USD ($)
Mar. 31, 2017
$ / shares
shares
Shares Vesting Based On Relevent Performance Period At March 31, 2017      
Restricted Stock Unit Activity - Other disclosures      
Number of units reached maturity 183,322    
Vesting percentage 163.00%    
Options issued (in shares) 299,641    
Restricted stock units      
Restricted Stock Unit Activity - Other disclosures      
Share-based compensation expense | $ $ 18.6 $ 15.4  
Restricted stock units | Time Based Vesting      
Non-vested restricted stock unit activity      
Outstanding at the beginning of the period (in shares) 3,307,577    
Granted (in shares) 1,424,678    
Vested (in shares) (691,507)    
Forfeited or cancelled (in shares) (196,056)    
Outstanding at the end of the period (in shares) 3,844,692   3,307,577
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) | $ / shares $ 22.57    
Granted (in dollars per share) | $ / shares 26.09    
Vested (in dollars per share) | $ / shares 20.44    
Forfeited or cancelled (in dollars per share) | $ / shares 22.51    
Outstanding at the end of the period (in dollars per share) | $ / shares $ 24.26   $ 22.57
Weighted-average remaining contractual term      
Weighted average remaining contractual term (in years) 2 years 7 months 3 days   2 years 5 months 12 days
Restricted Stock Unit Activity - Other disclosures      
Granted (in shares) 1,424,678    
Aggregate fair value of restricted stock units granted | $ $ 37.2    
Restricted stock units | Performance stock      
Non-vested restricted stock unit activity      
Outstanding at the beginning of the period (in shares) 732,711    
Granted (in shares) 421,918    
Additional performance shares (in shares) 94,775    
Vested (in shares) (252,760)    
Forfeited or cancelled (in shares) (13,498)    
Outstanding at the end of the period (in shares) 983,146   732,711
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) | $ / shares $ 20.89    
Granted (in dollars per share) | $ / shares 26.70    
Additional performance shares (in dollars per share) | $ / shares 19.46    
Vested (in dollars per share) | $ / shares 19.46    
Forfeited or cancelled (in dollars per share) | $ / shares 18.91    
Outstanding at the end of the period (in dollars per share) | $ / shares $ 23.64   $ 20.89
Weighted-average remaining contractual term      
Weighted average remaining contractual term (in years) 1 year 7 months 13 days   1 year 1 month 17 days
Restricted Stock Unit Activity - Other disclosures      
Granted (in shares) 421,918    
Aggregate fair value of restricted stock units granted | $ $ 11.3    
Options issued (in shares) 94,775    
Restricted stock units | Shares Vesting Based On Relevant Performance Period At March312018 [Member]      
Restricted Stock Unit Activity - Other disclosures      
Number of units reached maturity 277,792    
Vesting percentage 200.00%    
Options issued (in shares) 555,584    
Restricted stock units | Performance Shares Vesting Total Shareholder Return      
Non-vested restricted stock unit activity      
Granted (in shares) 217,784    
Restricted Stock Unit Activity - Other disclosures      
Granted (in shares) 217,784    
Aggregate fair value of restricted stock units granted | $ $ 6.1    
Restricted stock units | Performance-based Restricted Stock Units Vesting In Two Equal Tranches [Member]      
Non-vested restricted stock unit activity      
Granted (in shares) 87,184    
Restricted Stock Unit Activity - Other disclosures      
Granted (in shares) 87,184    
Aggregate fair value of restricted stock units granted | $ $ 2.1    
Number of tranches | tranche 2    
Restricted stock units | Performance-based Restricted Stock Units Vesting In Three Equal Tranches [Member]      
Non-vested restricted stock unit activity      
Granted (in shares) 116,950    
Restricted Stock Unit Activity - Other disclosures      
Granted (in shares) 116,950    
Aggregate fair value of restricted stock units granted | $ $ 3.1    
Number of tranches | tranche 3    
Restricted stock units | Shares Vesting Based On Relevent Performance Period At March 31, 2017      
Restricted Stock Unit Activity - Other disclosures      
Number of units reached maturity     157,985
Vesting percentage 160.00%    
Options issued (in shares) 252,760    
Restricted stock units | Share-based Compensation Award, Tranche One      
Restricted Stock Unit Activity - Other disclosures      
Future share-based compensation expense expected | $ $ 22.6    
Restricted stock units | Share-based Compensation Award, Tranche Two      
Restricted Stock Unit Activity - Other disclosures      
Future share-based compensation expense expected | $ 33.8    
Restricted stock units | Share-based Compensation Award, Tranche Three      
Restricted Stock Unit Activity - Other disclosures      
Future share-based compensation expense expected | $ 23.3    
Restricted stock units | Share-based Compensation Award, Tranche Four      
Restricted Stock Unit Activity - Other disclosures      
Future share-based compensation expense expected | $ 11.2    
Restricted stock units | Share-based Compensation Award, Tranche Five      
Restricted Stock Unit Activity - Other disclosures      
Future share-based compensation expense expected | $ $ 1.5    
Minimum | Restricted stock units | Performance Shares Vesting Total Shareholder Return      
Restricted Stock Unit Activity - Other disclosures      
Vesting percentage 0.00%    
Minimum | Restricted stock units | Performance-based Restricted Stock Units Vesting In Two Equal Tranches [Member]      
Restricted Stock Unit Activity - Other disclosures      
Vesting percentage 0.00%    
Minimum | Restricted stock units | Performance-based Restricted Stock Units Vesting In Two Equal Tranches (Second Tranche) [Member]      
Restricted Stock Unit Activity - Other disclosures      
Vesting percentage 0.00%    
Minimum | Restricted stock units | Performance-based Restricted Stock Units Vesting In Three Equal Tranches [Member]      
Restricted Stock Unit Activity - Other disclosures      
Vesting percentage 0.00%    
Maximum | Restricted stock units | Performance Shares Vesting Total Shareholder Return      
Restricted Stock Unit Activity - Other disclosures      
Vesting percentage 200.00%    
Maximum | Restricted stock units | Performance-based Restricted Stock Units Vesting In Two Equal Tranches [Member]      
Restricted Stock Unit Activity - Other disclosures      
Vesting percentage 75.00%    
Maximum | Restricted stock units | Performance-based Restricted Stock Units Vesting In Two Equal Tranches (Second Tranche) [Member]      
Restricted Stock Unit Activity - Other disclosures      
Vesting percentage 150.00%    
Maximum | Restricted stock units | Performance-based Restricted Stock Units Vesting In Three Equal Tranches [Member]      
Restricted Stock Unit Activity - Other disclosures      
Vesting percentage 300.00%