STOCKHOLDERS' EQUITY: - Non-vested performance-based restricted stock unit activity (Details) - Restricted stock units - Performance stock - $ / shares	6 Months Ended	12 Months Ended
	Sep. 30, 2019	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Non-vested restricted stock unit activity
Outstanding at the beginning of the period (in shares)	394,188	394,188
Granted (in shares)		202,818	534,438	389,065
Forfeited or cancelled (in shares)		(51,560)
Outstanding at the end of the period (in shares)		545,446	394,188
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)	$ 43.88	$ 43.88
Granted (in dollars per share)		60.65
Forfeited or cancelled (in dollars per share)		34.42
Outstanding at the end of the period (in dollars per share)		$ 51.01	$ 43.88
Weighted average remaining contractual term (in years)	3 years 2 months 23 days	2 years 2 months 26 days